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PNP Germanium Transistor Buffer

Basic Info and Mods

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.

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).

A germanium transistor is provided with each buffer pcb that is sold here


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 is built on the AMZ buffer board. PCB is 1.5" x 1.25" (38,1mm x 31,75mm).


Parts List

R1, R5 - 1M
C1 - 0.22uF
R2, R3 - 220k
R4 - 10k
C2 - 1uF
R5 - 1M
C3 - 47uF
Q1 - PNP germanium transistor (AC128 or similar)

All resistors are 1/4w and capacitors are in uF.

ROHS Compliant
Here is the blank buffer pc board with the parts layout superimposed over it.

You can follow the signal path through the pcb by following the traces that can be seen through the translucent board.

A complete description of each part's function:

R1 - input pulldown resistor
C1 - AC couples signal to transistor base
R2, R3 - set bias operating point for transistor
R4 - emiiter reference for output (tied to +9)
C2 - AC couples signal to the output
R5 - output pulldown resistor
R6, C3 - power supply filter for hum prevention
Q1 - PNP germanium transistor (AC128 or similar)


The 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.



Once all of the parts are soldered on the board, it should look something like this.

All of the connections to the board are along the bottom edge.

Note the tab on the metal case of the AC128, which indicates the emitter pin.


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.


MODIFICATIONS

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

You will note in the picture immediately above, the color bands on the R2 and R3 resistors show that 470k was used on the demo board.

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

Click here to purchase!


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