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PNP Germanium Transistor Buffer

Basic Info and Mods

NOTE: This project originated as an experiment in how to use the germanium transistors that are left over when sorted for leakage and gain. It is not hi-fi and I would not recommend it for design into a new commercial product!

The AC128 is a great sounding germanium transistor but if you buy many of them, there will be a large number left over that have too much base-to-emitter leakage to be useful in a voltage amplifier circuit.

On the Small Bear Fuzzface FAQ, there is a drawing that illustrates how to measure transistor leakage near the end of the page.

Ge transistors that have too much leakage for use in small signal amplifiers can be used as diodes. The base-to-emitter junction can be used like a signal diode; the emitter is the anode and the base is the cathode (for a PNP transistor).

In a search for something to do with the leftover leaky transistors, I came up with the idea of using them in a basic emitter follower buffer configuration. The tiny leakage current is immaterial in this case and has no impact on the sound of the circuit.

The result is the schematic that is seen here to the left. It is a basic bipolar transistor buffer, but since the AC128 is a PNP transistor, the circuit has been modified slightly to work on the typical negative-ground power supply that is most common in guitar pedalboards.

This is the schematic for the circuit that can be built on the AMZ multi-purpose board (shown below), or even a piece of perfboard.

The transistor pins are identified by looking at the bottom of the transistor as shown here.

They are arranged in a triangle configuration with the base connection in the middle. If you place the transistor as shown here, the emitter will be to the right (base is the placed at the lowest point). There may also be a small metal tab on the case, and it will be closest to the emitter lead.

Here is a bad picture of the output of the AC128 buffer that is described above. It is a 2v pk-pk, 100kHz triangle wave going through the exact setup shown in the schematic. The response is flat from 10 to 100kHz and beyond - excellent performance. This test was run with the leaky PNP transistor operating on a 9v battery.

I tested 10 leaky AC128s and got identical performance from all of them, with current leakage measured in the range of 400uA to 700uA.


Even though the input impedance is limited by the gain of the transistor, a common mod for the buffer would be to increase the input impedance slightly. This is done by raising the values of R2 and R3. On the modified version to the left, those resistors are now 470k.

You might even use 1M resistors for R2 and R3 to see if it makes a bit brighter tone that is compatible with your rig. The impedance is lower than what you can get with a high gain silicon transistor or a jfet, but should be adequate for guitar use.

The remainder of the mods are on a page that is
reserved for those who buy the pc board.

Click here to purchase! A Ge transistor is included with the pcb.

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