Three-Stage BJT Amplifier with 100V/V Gain

Thread Starter


Joined Nov 30, 2006

I designed a three-stage BJT amplifier with common emitters on the first and second stages and a common collector on the last. I was able to design the circuit so that the 50mV, 10kHz input was amplified to 5V for a gain of a 100.

The problem I am having is the total harmonic distortion. The design specifications said it has to be some 0.x%, but I am getting around 247%. In Pspice, under the transient option, I enabled Fourier, set the center freq to 10khz and had 9 total harmonics. I set the variable at the output node the resistive load, simulated, and got this extremely high percentage. My output is a smooth curve in Pspice. Also, I built the circuit in lab and I get the 100 gain with a smooth curve on the oscilloscope.

What are ways to lower the THD? Is the problem with my design or did I simulate incorrectly? I attached a picture of my circuit with node voltages. Any help will be appreciated!
Thank you.


Joined Jan 28, 2005
Greetings okal,

I have taken a look at your schematic and it is my guess that the second stage in your amplifier is the most likely culprit to be introducing distortion into the signal.

It looks like the base bias point as set by the resistor divider R3 and R4 is placing the voltage at Q7's emitter too positive. The higher positive voltage on the emitter means that the negative signal swing at Q7's collector is causing Q7 to begin to enter the saturation region of Q7.

I suggest you adjust the DC bias point of Q7 more negative by at least a couple of volts if not more. That will mean that you will need to change the 10K resistor in Q7's emitter network also. This should result in an improvement in the signal headroom in the second stage. You should see a dramatic improvement in the THD with this adjustment.

Good Luck,