Introduction: Bad Transistor Amplifier


This Instructable shows what you should not do in analogue electronic circuit design.

Some people think that putting diodes as feedback will allow biasing of BJT transistors without a variable resistor to adjust the transistor collector voltage. This Instructable will show why this circuit is wrong.

In this circuit, I also connected a low feedback resistor because even 100 kohm resistor might result in small collector current and collector voltage almost equal to the supply voltage.


You need any simulation software. Online or installation software. I used old PSpice student edition version 9.1

Step 1: Simulations

From simulations on the graph (second picture), you can calculate the voltage gain:

Vc2(peak to peak) / Vc1(peak to peak) = 0.25 V / 0.02 V = 0.125 V / 0.01 V = 12.5.

This is a very low gain. This low gain is caused by:

- mostly low feedback resistance,

- and also the presence of a feedback diode.

This circuit also has non-linear AC gain due to feedback diodes. You can see that if you increase the input amplitude. BJT transistors are used in most integrated circuits (ICs) to achieve a linear gain which FETs (Field Effect Transistors) do not allow.

I also tried making this circuit and I was not getting much gain at all.

Increasing Rb1 or Rb2 resistors from 1 kohm to 10 kohm will increase the output amplitude due to less loading affects. However, high Rb1 / Rb2 values will reduce the voltage entering the base of the transistor. Maximum power transfer rule is when the source impedance is equal to the load impedance.