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Type 2N107 Transistor Circuits for Experimenters-Amateurs

Type 2N107 Transistor Circuits for Experimenters-Amateurs

The following article is Steven Coles' Experimenter's notes on GE's 2N107 projects.

Discussion and Notes

For the most part these notes also apply to versions of the circuits appearing in GE's thin paper (packed with 2N107 transistors) and in the GE Transistor Manuals.  Some portions may be useful with early projects from other transistor manufacturers.


Excepting those located near high-power transmitters (perhaps 5 miles), simple-receiver listeners need 5 to 30 meter long (15 to 100 foot) wire antennas.  These antennas require low-resistance earth grounds for optimum operation.  More complicated two and three transistor receivers of the regenerative, reflex and superheterodyne types often give satisfactory performance using a loopstick (ferrite-rod coil) or indoor loop as an antenna.

If you have a Miller 6300 loopstick or equivalent known brand and part number loopsick, it has value for restoring vintage transistor receivers.  Some choose restoration as their primary motivation for learning transistor electronics.

Note 1 – A Miller 6300 work-alike for hobby (but ugly for restoration) consists of 3.5 meters (12.5 feet) litz wire (preferred) or AWG 26 to 30 magnet wire scramble wound onto a salvaged ferrite rod (from a clock-radio or boom-box receiver).  A tap is a 1.5-cm (0.5-inch) twist left sticking out from the main winding.  To make your ferrite-rod coil useful for many experiments, make one tap at each of 10 turns, 37 cm, 75 cm and 150 cm.  You can later remove the insulation and tin whichever taps become needed.  Leaving 10 cm (4 inches) wire as leads, hot glue the winding at each end.

Note 2 – Instead of buying or making a Miller 6300 work-alike, an entire loopstick plus variable capacitor can be savaged from a defunct clock-radio or boom-box receiver.  These tuned circuits have poorer selectivity (the ability to separate two stations of nearly the same frequency).  How well they work depends on selectivity needs at the listener's receiving location.  When poor selectivity results in two stations heard at the same time, a “wave trap” between the antenna and receiver usually solves the problem (at the cost of an extra tuning step when changing stations).

Loop Antennas

Note 3 – Apartment dwellers and others with external-antenna restrictions can receive with a 100-cm (40-inch) square, spaced-wire loop.  An “X”-shaped frame supports the wire at each corner.  Frame ends (wire corners) each have seven notches spaced about 1-cm apart.  Twenty-four meters of stranded AWG #20 to 24 wire is wound onto the frame forming 5 turns.  If the resulting loop does not tune low enough with the available variable capacitor, 1 or 2cturns may be added later.  Being highly directional, the loop must be rotated for maximum reception.  As the loop will be used indoors, some builders style it like furniture or an art object.  An example can be found at Make A Radio.

Assembly Methods

Simple circuits can be assembled in numerous including brass machine screws through plastic or wood, driving brass-plated tacks into wood and soldering to the tacks, prototyping boards (for short-term use), Manhattan and ugly.  While senior builders (like the 68-year old writer) become insecure at surface-mount's mention, younger builders who don't know any better effortlessly assemble on general-purpose surface-mount boards.  Manhattan details: Manhattan 1 and Manhattan 2

Aunts and Cousins:  Many hobbyists main source for defunct clock-radios, boom boxes, 49-MHz walkie-talkies, 49-MHz cordless phones, radio-controlled toys, baby monitors, computer speakers, game modulators and wall-wart power supplies.  When listing what you wish relatives and friends to save, care in excluding digital cameras, cell phones, TVs and computer monitors makes the method work.  Once disassembled, these latter items present huge disposal problems.  The component networking folks carefully exclude technophobic and attention deficit friends and relatives from their sources.

Batteries and Cells:

Any circuit in Type 2N107 Transistor Circuits for Experimenters-Amateurs can be powered from “AA” or “AAA” cells.  These circuits do not need types C and D cells.  Put types C and D cells in equipment genuinely needing the higher ampere hours.  Portable florescent light inverters usually do need the larger and heavier cells.

Current, Voltage, Power, Time and Energy:  If you don't want the science and math, skip this paragraph.  Ampere hours ratings allow comparing batteries supplying the same voltage.  But even middle-school science and shop teachers, know ampere hour ratings don't work for comparing batteries of different voltages.  Energy gives much more useful information.  To find a battery's energy, follow these steps:  1.  If the life is given in milliamp hours, divide by 1,000 to get ampere hours.  2.  Multiply ampere hours by volts to get watt hours.   3.  Then divide by 3,600 (the number of seconds per hour) to get joules.  One joule supplies 1 watt for 1 second.  [Can't believe I wrote that off the top of my head!  Someone check my math.]

Note 4 – Many hobby stores sell holders for two AA or two AAA cells.  Holders may sometimes be cut from crushes motorized toys.  Holders can be made from hobby tubing (just big enough for the cells to fit in), flat brass strips and elastic bands.

Note 5 – Cell producing dead indications in digital products will often operate a two-transistor analog circuit for 12 or more hours.  Consider recycling cells from digital to hobby transistor projects.  For another dead-cell-operated device surf the Internet for “joule thief.”

Note 6 – Hearing-aid, watch, coin and button cells producing approximately the correct voltage will also power transistor projects.

Note 7 – Also consider using lemon cells, potato cells, salt cells, earth cells, voltaic piles and energy-harvesting devices.


Most active electronics devices (such as bipolar junction transistors, field-effect transistors and thermionic vacuum tubes) can operate in multiple ways.  Engineers and technicians describe their circuits as operating in simple-switching, class A, B, C, D, E, F, constant-current or constant voltage modes.  In Type 2N107 Transistor Circuits for Experimenters-Amateurs the transistor's DC base current sets the operating point and mode.

In Type 2N107 Transistor Circuits for Experimenters-Amateurs most amplifiers operate in class-A mode.  In class-A the transistor remains in linear operation for the entire signal cycle.  In the “Loud Speaker Audio Amplifier” the two output transistors operate in class-B (actually a variation called class-AB).  In class-B operation one transistor remains in linear operation throughout the signal's positive half cycle and the other transistor remains in linear operation during the negative half cycle.  In class-B a transistor not in linear operation stays in cut-off (almost zero collector current) until returning to linear operation.  The “Loud Speaker Audio Amplifier” output stage is also said to be of the push-pull type.

The two-transistor receiver's input transistor also operates class-B.  In this receiver the class-B operation achieves “envelope demodulation” (also called detection). 

Note 8 – Excepting the “Code Practice Oscillator,” Type 2N107 Transistor Circuits for Experimenters-Amateurs using class-A operation set the operating mode by a single resistor from the negative battery terminal to the transistor's base terminal.  For longest battery life, this “base-bias” resistor should be set to the largest value producing clean-sounding (low distortion) sound.  More mathematics hobbyists can usually skip:  The “Simple Radio Receiver” has a 3-volt battery.  A typical germanium transistor produces a 0.2-volt drop across its base-emitter junction.  That leaves 2.8 volts across the 220,000-ohm resistor.  Ohm's law states the current through a resistor is the voltage across the resistor divided by the resistor's value.  So 2.8 volts divided by 220,000 ohms gives about 0.000013 amp.

Note 9 – Type 2N107 Transistor Circuits for Experimenters-Amateurs gives no resistor power ratings.  When written, no rating meant use 0.5 watt resistors.  But now ¼ and 1/8 watt ratings have become common.  For DC, power equals voltage multiplied by current.  For the example in Note 8 we have 2.8 volts times 0.000013 amp equals about 0.000036 watt.  Almost any resistor used by hobbyists will handle that power.

Capacitors, Fixed-Value

When germanium hobby transistors were widely used, capacitors were called “condensers.”  “Condenser” dates to a pre-electron-discovery period when electricity was though a fluid.  The designation mmF (mico-micro farad) has been replaced by pF (picofarad).

During germanium-transistors' peak years, low-value fixed capacitors were types suited to any frequencies up to 30-MHz and often higher.  Since then low-value fixed capacitors have found applications in lower-frequency equipment.  Inexpensive ceramic disk capacitors (as sold for hobby electronics) often have too high dielectric absorption for use above 300-kHz.  When building radio-frequency equipment, be sure to use capacitor types suited to radio frequencies.

Note 10 – When in doubt, trust capacitors salvaged from dead receivers' RF (and FM receivers' IF) sections in preference to hobby-grade capacitors.

On the other hand, germanium hobby projects usually coupled audio signals to a transistor's base with 0.047 μF (microfarad) capacitors.  Higher-valued capacitors were bulky and expensive.  By using 22 μF to 47 μF aluminum electrolytic or tantalum electrolytic capacitors, bass response can be greatly improved.

Note 11 – When using electrolytic capacitors in place of earlier types, capacitors with sufficient voltage ratings must be selected.  If in doubt, use capacitors with voltage rating exceeding the largest battery voltage (3 volts for most Type 2N107 Transistor Circuits for Experimenters-Amateurs circuits and 6 volts for the “Loud Speaker Audio Amplifier”).  Be sure to install electrolytic capacitors according to circuit-diagram or layout polarity.  [circuit diagrams need polarity added.]

Note 12 – In some cases using larger capacitors may pass objectionable hum pickup.  Before giving up on the larger capacitors, try connecting a disposable aluminum pie pan via a wire to the amplifier's or receiver's common (ground) terminal.  Place the under, but insulated from the electronics.  If this corrects the hum, the equipment needs a ground plane (a disposable aluminum pie pan cut to fit under the equipment).  Also see “Hum” below.

Capacitors, Variable

Highly-selective contest receivers use top-end air-insulated variable capacitors for their high Q (quality in resonant and filtering circuits – explanation beyond scope here) advantage giving slightly better selectivity (ability to hear a weak station at a frequency close to a strong station's).  Transmitters may require air-variable capacitors for their higher voltage ratings, lower power dissipation and “self healing” after arc over.  Air-variable capacitors of sufficient quality for a contest receiver may cost over US$50.  (Prices are as of 2015 with both air-variable and polycap prices rising rapidly.)  A contest receiver may require three such capacitors.

Polycaps (plastic-insulated variable capacitors) usually cost under US$10.  Polycaps are lighter and smaller than air-variable capacitors.  Unlike air-variable capacitors, polycaps protect internal workings from bumps, dust and insects.  Polycaps intended for the USA's FM broadcast band have sufficiently low dielectric absorption to allow local oscillator operation to above the needed 118.7 MHz frequency.  Plastic-on-metal friction limits polycap longevity.  A low-price, frequently re-tuned polycap may wear out in five years.  At the other extreme many polycaps installed in early transistor receivers continue operating almost like new today (2015).  When soldering polycaps, care must be taken to avoid overheating polycaps when soldering to them.

Note 13 – When using a salvaged polycap in a receiver, it will have a maximum capacitance approximately 180-pF.  Usually, a salvaged polycap has four sections.  The remaining sections have maximum values around 80 pF, 30 pF and 30 pF (the latter two for the FM broadcast band).  When paralleled, the four sections provide about 320 pF – close enough for Type 2N107 Transistor Circuits for Experimenters-Amateurs receiver projects.

Note 14 – Some with shop skills and tools (even this fool lacking both) assemble variable capacitors described in 1915-1925 vintage electronics books and magazines.


Some transistor receiver builders participate in 1AD (one active device) receiver contests.  Contest winners have polished their receiver-building skills through constructing many receivers.  For some radio builders contesting provides the primary motivation.

Current Gain

Common-emitter DC current gain equals the collector current divided by the emitter current.  Usually, germanium transistors have common-emitter DC current gain (HFE, Beta, β or AI) between 20 and 100.  Because current gain divides one quantity by another having the same units, current gain itself has no units.

Note 15 – From the discussion, collector current equals base current times common-emitter current gain.  Let's say the germanium-transistor in Note 8 has a current gain of 40.  Then its collector current equals 40 times 0.000013 amp which equals 0.00052 amp.


The “Simple Radio Receiver” uses a crystal diode as an envelope demodulator.  Cat whisker on crystal (galena, iron pyrite, polycrystalline silicon. . .), germanium (1N34, 1N64, 1N82, 1N270. . .) and Schottky-barrier signal diodes interchange without circuit modifications. Electronics Goldmine sells single-crystal silicon wafers which can also be tried. 

Distant Stations (DX)

In the high-frequency (HF, aka shortwave, 3-30 MHz) spectrum DX usually means listening to (or communicating with) stations transmitting from outside the listener's country.  In the medium wave (300 kHz to 3 MHz) and the very-high frequency bands (30 to 300 MHz) DX usually medians listening to stations at distances for which conditions seldom allow easy reception (perhaps 150 kilometers or more).  DX enthusiasts usually keep a log of stations heard and may supply the station with signal reports in hope of receiving a physical or electronic “QSL” card.  DX listening with a DIY receiver motivates some radio builders.


Some electronics engineers and technicians include hobby electronics within their self education.  When we scratch a little fresh paint off new technological compatibilities 97+% we find old foundation technologies in new combinations or one teaspoon completely new technology combined with one cup old foundation technology. 

Any government wishing technological (including innovative manufacturing techniques, products, strong international trade and employment) makes its 20 to 80 year old training, procedure and selected equipment manuals available through means understood by public and business librarians.  Such a government creates incentives for businesses to release their classic manuals, applications notes and even hobby publications through the national library's on-line system.

Frequency Terminology

Frequency terminology has changed since the germanium period.

Cycles per second (cps) has become hertz (Hz).

Kilocycles per second (kcps or kc) has become kilohertz (kHz).

Megacycles per second (Mcps or Mc) has become megahertz (MHz).

Gigahertz (GHz) has come into common usage.

Note 16 – The kilo, mega and giga prefixes carry the same meanings they do in the metric system.  Therefore, a station transmitting at 1,200 kHz will sometimes be said to be transmitting at 1,200,000 Hz or 1.2 MHz.


Improving grounds makes such great reception improvements some early radio experimenters fooled themselves into thinking radio traveled mainly through the earth.

Note 17 – Traditionally, radio builders living where cold water plumbing remains conductive into the earth make a ground by cleaning a spot on a cold water pipe and attaching a ground clamp (purchased from a hardware store) to which they connect the ground wire.  If they dwell in a plastic-plumbed structure or have one lacking plumbing near the listening point, they drive a ground rod (hardware store again) into the earth and attach the ground clamp to the ground rod.

Note 18 – Where a wiring plus pipe run from the receiver to earth exceeds one eighth wavelength at the highest frequency of interest, a capacitive ground may perform better than a conductor-connected ground.  For a trial, fasten three disposable aluminum pie pans together with machine screws and nuts.  Connect a 1.5 meter wire to the pie pans and connect the wire's free end to the receiver's ground terminal.  Place the pans on the floor (in a safe place – let's not slip and fall from stepping on them).  Try some listening.


The 1960s saw consumer headphones transition from a standard impedance magnitude of 2000-ohms to a standard impedance of magnitude 32-ohms.  Ear buds also present a 32-ohm impedance to the amplifier.  The lower impedance operates poorly or not at all when placed in the circuits under discussion.

Note 19 – The newer headphones are used with a 1000-ohm-to-8-ohm matching transformer.  The 1000-ohm winding connects where the 2000-ohm headphones would have connected and 32-ohm headphones connect to the 8-ohm winding.

Note 20 – Both Rochelle-salt and ceramic piezoelectric earpieces can sometimes be purchased.  Even though externally they all appear identical, the efficiency and longevity vary greatly.  They will not operate simply connected into the Type 2N107 Transistor Circuits for Experimenters-Amateurs circuits.  However, they will operate with a 22,000-ohm [value?] resistor in parallel.  The base-resistor value will need adjustment (usually upward) when using piezoelectric earpieces in simple transistor circuits.  Thermal runaway can not damage a transistor with a high resistance in series with its collector lead.

Note 21 – Some radio builders have found telephone receivers (earphones) with impedances high enough for direct use in circuits from the booklet under consideration.

Hum:  Usually, hum gets into hobby electronics via antenna, ground or microphone wires or from a badly-designed mains-energized battery eliminator.  See also Note 12 above.

Note 22 – If a DIY receiver hums, try placing small value (220 pF) capacitors in the antenna and ground leads.

Note 23 – If hum enters an amplifier from a microphone, be sure the shield is properly connected.  If the microphone is a balanced type, be sure the a balanced-to-unbalanced transformer is properly connected between the microphone cable and the amplifier input.


Hobbyists often ease learning by keeping component layouts similar to the schematic wiring diagrams.  Layouts can be drawn on legacy graph paper or on computer screens using Tube Pad Virtual Breadboard with an image editing program. 

Note 24 – Because beginners' books and hobby magazines presented the circuits under discussion in more detail, layouts often appeared as photographs or drawing in these publications.  See American Radio History

Loading Coils (for wire antennas)

When the antenna wire must be less than one-fourth wavelength long, loading coils greatly improve reception.  Antenna tuners do even better.  But Antenna tuners require more parts and more skill to use.  The loading coil is connected between the antenna lead in and the receiver's antenna terminal.

Note 25 – Usually, a loading coil is a variable coil having the same inductance as the tuning coil plus some taps for lower inductance ranges.  For commonly-needed taps see Note 1.


Hobbyists seldom buy microphones.  Instead they use headphones or speakers as microphones.  Electronic components and circuits that work in reverse are said to have “reciprocity.”  Other reversible components and circuits include light-emitting diodes, transformers and passive impedance-matching circuits.  Of course, hobbyists and students also obtain microphone elements from obsolete copper-connected telephones and defective cellular telephones.  Most microphones have low output voltage.  Therefore, they require a preamplifier between the microphone and the power amplifier.

Note 26 – Often in microphone amplifier combinations adding an impedance-matching transformer corrects slightly inadequate gain (volume).  When this works the circuit needs no additional transistors and draws no or little additional power.  However, audio transformers of the correct impedance ratio may be hard to find, expensive, bulky or frequency limiting.  Therefore, electronics hobbyists weight the benefits of an added matching transformer verses  additional transistor gain stage.  For mass production, the cost-benefit curves for the two options crossed during the 1960s.

Restore or Salvage

If you come by a receiver or other consumer-electronics product containing germanium transistors, it's likely a collectible model.  If the case remains in good condition or can be restored with craft-store casting and molding products and the circuit board has not been excessively damaged, consider restoration.  Certain germanium-transistor computer cards have historic value.

Shortwave Listening (SWL)

By changing the tuned circuit and using the best available antenna and ground the “Two-Transistor Radio Receiver” should produce some reception to about 6 MHz.  Shortwave reception on such a receiver usually requires a loading coil.  In the 1.7 to 6.0-MHz range the amateur-radio AM networks (a little below 4 MHz) and the 49-meter international broadcasts particularly interest shortwave listeners.  Language students in the USA's southwest include Spanish and in the New England and Midwest states include French in their reliable shortwave listening.  As presented in the booklet the “Two-Transistor Radio Receiver” operates only with amplitude-modulated (AM) signals.  It is not intended for radiotelegraph, suppressed-carrier or digital signals.

Component Substitutions

As transistor circuit components had not become standardized and mail-order was slow, 1950s and 1960s hobbyists substituted whatever components could be purchased locally, salvaged from defunct equipment or built from scratch.

Note 27 – Almost any PNP germanium junction transistor intended for audio preamplifier or audio driver applications and having a maximum power dissipation rating of 50 milliwatts or more can be substituted for a 2N107. 

Thermal Runaway

Any bipolar (manic/depressive) junction transistor can enter thermal runaway.  Rushing to meet publication deadlines, writers sometimes neglect thermal runaway checks.  Therefore, a builder using hard-to-obtain or expensive transistors does well to include a minimal check.  Good biasing prevents thermal runaway.  However, simpler biasing preventing damage may suffice for hobby projects.  It may even provide more learning.

According to the power-transfer theorem, a transistor in a common-emitter configuration draws maximum power when its collector voltage equals half the supply voltage.  This gives the messy-looking, but conceptually simple formula

Pc = Vc*Ic = (Vcc/2)*((Vcc-0.5*Vcc)/Rc))


Pc = (Vcc^2)/(4*Rc)

Which rearranges to

Rc >= (Vcc^2)/(4*Pc)

Note 28 – To protect a 50 mW transistor from damage, the series resistance must be equal to or larger than values given below.

Battery            Resistance


1.5V                15 ohms

3.0V                63 ohms

4.5V                150 ohms

6.0V                270 ohms

9.0V                560 ohms

A large collector or emitter resistor does not prevent thermal runaway.  The circuit may still lock up in a “soft” failure.  However, the transistor will remain within its dissipation limits.  Remaining within dissipation limits does not guarantee a transistor will remain within its voltage and current limits.

Where a load provides too little resistance (frequently occurring with lamps, relays, solenoids, speakers, transformers, . . . ) a series resistor can sometimes be added.


Some transistor circuits use transformers for impedance-matching (Two-Transistor Radio Receiver) and phase splitting (LoudSpeaker Audio Amplifier).

Note 29 – Audio transformer type numbers have yet to become standardized.  Electronics builders collect audio-transformer catalogs for choosing transformer substitutions.

Stancor UM-112 → Triad SP4

Triad A-81X→Triad TY-142P or Triad SP20, SP21, SP22

Triad S-51X →

Tamaru and Xicon manufacture miniature audio transformers.  At the time of writing Mouser Electronics carries these transformers.

Note 30 – An FM-clock-radio 10-volt power transformer can be re-purposed as a (120-to-10) squared equals 144-to-1 impedance-ratio audio transformer – just an example.  Seemingly no standard receiver power-transformer ratio exists.  Such a transformer might serve in the Type 2N107 Transistor Circuits for Experimenters-Amateurs “Two Transistor Radio Receiver.” The less-common 24-volt power transformer can be re-purposed as a (120-to-24) squared equals 25-to-1 impedance-ratio audio transformer.  The latter might be used as a (bulky) transistor interstage transformer.

Tuned Circuits

For the frequencies in Type 2N107 Transistor Circuits for Experimenters-Amateurs and many more, tuned circuits usually consist of a variable-inductance coil and a fixed low-value capacitor (typically 330 pF) or a fixed inductance and a variable capacitor (typically 15 to 180 pF or 25 to 350 pF).  The two components are connected in either series or parallel (depending on he use).  Both Type 2N107 Transistor Circuits for Experimenters-Amateurs radio receivers show tuned circuits in the parallel connection.  For commonly-needed taps see Note 1.

Note 31 – As hobbyists, students and technicians so frequently need tuned circuits, they often arrange the two components on a small board with a switch or jumper wire so the components may be connected either series or parallel.  The tuned circuit will then serve as a wave trap or, if calibrated, and absorption wave-meter.  If the coil is wound with a second winding of 10 turns and suitable taps, the tuned circuit may be used in tickler-coil oscillator, Hartley oscillator, regenerative receiver, preselector, Q-multiplier and transmitter power amplifier studies.  It may also be used for couple radio-frequency energy (RF oscillator outputs) to circuit having input impedances differing from the source (oscillator) impedance.

Frequency & Wave Length

Most electronic hobbyists use a chart or just remember the wave lengths for the frequencies they most use.  Those who like to calculate use λ = C/F where  λ equals wavelength in meters, C equals the speed of electromagnetic radiation (including radio and light) in free space (300,000,000 meters per second) and F equals frequency in Hz.

Note 32 – Using the formula a small table might include

Frequency                 Wavelength   Quarter Wavelength

300,000 Hz                1,000 m          250 m

540,000 Hz                ~556 m           ~139 m

1,000,000 Hz            300m              75m

1,700,000 Hz            ~176m            ~44m

1,875,000 Hz            160m              40m

3,000,000 Hz            100m              25m

3,750,000 Hz            80m                20m

4,000,000 Hz            75m                18.75m

5,000,000 Hz            60m                15m

6,000,000 Hz            50m                12.5m

6,125,000 Hz            ~49m              ~12.25m

7,000,000 Hz            ~42.9m           ~10.7m

7,500,000 Hz            40m                10m

Wave Traps

Parallel tuned circuits rejecting an unwanted frequency are called wave traps.

Note 33 – Receiver hobbyists often dig out an irreparable receiver when needing an AM broadcast-band wave trap.  They salvage the “antenna” tuned circuit (usually a loop stick and a polycap).  The first try reassembles the tuned circuit and places the resulting parallel capacitor-coil combination between the antenna and the receiver's antenna terminal (i.e. In series the the antenna lead-in).  If tuning this simplest trap does not sufficiently attenuate the unwanted signal, they disconnect the trap and wind 10-turns magnet wire around it.  They then place this new winding in series with the antenna lead in.  In the unlikely event this second arrangement does no suffice, the radio experimenter orders higher-quality wave-trap components or uses a directional loop antenna.