Hawes Mechanical Television Archive by James T. Hawes, AA9DT
Redesign Germanium Transistor Circuits, Part 1

Today... Build Germanium Transistor Projects
with Silicon Transistors!

Return with us now to the halcyon days of germanium!

Germanium transistor projects intrigued experimenters from the early days in the 1950s until the early 1970s. In the Seventies, the higher-grade silicon devices began to take over the hobby market. Hobbyists adapted, by attempting germanium-to-silicon conversions. Often, the germanium circuits didn't cooperate. Several difficulties occurred. Fortunately, each one had a solution. While the solution didn't always work, it usually did. This page describes the conversion process.

       Germanium transistors

Conversion Problems

Four Problems. In converting from germanium to silicon devices, you must solve four problems...

  1. Current gain. The rule of thumb is that silicon devices develop ten times the current gain of germanium devices. (This statement is only true of small-signal devices.)

  2. Leakage. Germanium devices often leak enough that they can bias themselves. Silicon devices hardly leak at all.

  3. Bias voltage. For Class-A operation, germanium devices require a no-signal bias of 0.3 volt. The equivalent Class-A bias for silicon devices is 0.7 volt.

  4. Silicon device shape might differ from the germanium device shape. The device pins might be in a different order.

Illustration: Man 
       contemplates problems

Conversion Solutions

  1. Current gain. In some circuits, more gain will be helpful. For example, the revised circuit might be more sensitive than the original circuit. But sometimes, limiting the gain (adding an emitter resistor, etc.) will be necessary.

  2. Leakage. Bias by leakage won't be possible with normal silicon devices. If the germanium circuit doesn't have bias resistors, you must add them. Otherwise, your silicon transistor won't work.

  3. Bias voltage. Correct bias is imperative. The main point of this page is rebiasing your transistor base-emitter junction. The correct bias will allow the device to amplify.
       Brainstorming solutions
  1. Silicon device shape. Package profiles have changed over the years. Expect the silicon device to be a different shape than the legacy germanium part. We're looking for reasonably similar behavior, not form. Often, the device pins will be in a different order. With persistence and ingenuity, you'll be able to fit the new device in the cabinet. You might have to shape the pins to fit the mounting holes in the PC board.

Universal Replacement Parts

For universal replacement parts, see our page Replacement Transistors, Capacitors & Diodes.

The table below offers help with typical conversion problems...

Conversion Method

Symptom Problem Solution (silicon circuit) See Figure
Transistor distorts, or doesn't come on. Improper bias for silicon.
  1. If no base resistor, use value: Base resistor Rb = (400X collector resistor Rc).

  2. If one base resistor Rb, change value: Rb= (400X collector resistor Rc).

  3. If voltage divider (Vd) bias: Increase value of resistor to common (Rb2) until resistor drops 0.7 VDC between base and emitter. (Double or triple resistor value in germanium circuit.)

  4. If the circuit has bias elements besides resistors, disconnect them. For example, some germanium circuits include thermistors in the bias network. (These might be in addition to bias resistors.)




Circuit blocks. Gain of silicon transistor is way too high.
  1. In one-resistor bias circuits, multiply base resistor Rb by about 20. Base resistor Rb should = (collector resistor Rc x 400). Formula only applies to circuits where Rb returns to power rail. See Figure 1.

  2. If base resistor returns to collector, instead of power rail (right): Rb= (Rc x 200). Rc is collector resistor.

Schematic: PNP amplifier with base resistor connecting to collector


Circuit distorts. Gain of silicon transistor is slightly high.
  1. Eliminate emitter bypass capacitor Ce.

  2. If no emitter resistor Re, add one. Resistor Re value should be 20% of collector resistor Rc value.

  3. Add emitter resistor Re. Then adjust base resistors Rb1 & Rb2 as necessary. (See next point.)

  4. Necessary Adjustments: •2-resistor circuits: Adjust resistor Rb2 value until it drops 0.7 VDC between base & emitter. •1-resistor circuits: Increase resistor Rb by one standard value until circuit operates properly.)




1 & 2

Circuit picks up radio. Too much high-frequency response.
  1. Solder a 0.001 µF, ceramic capacitor between the base and the collector. This part will likely squelch the unwanted signal.

  2. If not, replace the 0.001 µF cap with a 0.01 µF cap.
Schematic: Where to install 
               capacitor that will filter out radio signal

1 & 2

  1. PNP: Silicon replacement is 2N2907 or 2N3906. (Or: device with beta= 200).

  2. NPN: Silicon replacement is 2N2222 or 2N3904. (Or: device with beta= 200).

  3. You may use other devices. But in 1-resistor bias circuits, you must adjust the base resistor Rb value for correct bias. Device beta affects the value of this resistor. Typically [Rb= 2 x (beta x Rc)]. Rc= collector resistor value.

  4. Correct bias: DC, no-signal voltage on the collector should be about half of Vcc. (Example: Consider an NPN circuit running on 9 volts. The normal collector voltage with no signal on the base is 4.5 to 4.95 volts.)

 Fixed-bias circuits, adapting germanium to silicon circuit

•Figure 1. PNP amplifiers with 1-resistor bias

 Voltage-divider bias circuits, adapting germanium to silicon circuit

•Figure 2. PNP amplifiers with voltage-divider bias

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WARNING. The author assumes no responsibility for your success or failure in using methods on these pages. Further, the author neither makes nor implies any warranty or guarantee as to the accuracy or effectiveness of these methods. Proceed at your own risk.

CAUTION. Avoid using the conversion method on this page for direct-coupled preamplifiers, except for Darlingtons. To convert push-pull and complementary output circuits, see Page 6. These instructions on this page are for single-ended amplifiers that couple through a capacitor and resistor.

CAUTION. Avoid using one-resistor (fixed) bias with Darlingtons. With one-resistor bias, the device beta (current gain) determines the bias resistor value. Darlington beta can be in the thousands or even hundreds of thousands. For this reason, the bias resistor will be of impractical size.

EXAMPLE: You plan to use a Freescale MPSA13 Darlington. This device has a gain of 200K. The collector resistor is 4.7K. Your base resistor will then be 2(200K x 4.7K), or 1.88 billion ohms!

Schematic: Darlington with mammoth base resistor: Showing impracticality
                          of one-resistor bias for Darlingtons

• NOTICE. For adventurous souls who want to build the above circuit anyway: This example circuit probably wouldn't work, as the base current might be less than the device leakage current. Leakage current and beta differ from device to device. Beta also isn't a constant. It varies within any device depending on device current and operation frequency.

A potential use for this circuit is a temperature sensor. The circuit has a thermal stability factor (SF) of twice beta, or 400,000. A desirable SF is 20 or lower. For this reason, the circuit would be useless as an amplifier. But its thermal instability might make it valuable as a thermometer! I suggest substituting the reversed BE junction of a 2N3904 transistor for the implausible 1.8 GΩ resistor. See right for experimental circuit.

Schematic: Temperature sensor with Darlington and transistor with 
                          reversed BE junction for bias

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