I'm working on a project that requires me to design a multistage BJT amplifier.

The major design specs are as follows
• No more than 3 transistors (stages)
• Power supply is 15V
• No larger than 15kohm output impedance
• No less than 200kohm input impedance
• No larger than 2mA quiescent current draw
• No less than 8 V peak to peak output
• Gain must be 50 for the freq. range of 20Hz to 20kHz.

So far, I've been able to achieve all of these (as far as I know) using only two stages (CC followed by CE).

Schematic is attached

Since I have additional transistor stage that I am allowed to implement, I am curious to know if you have any suggestions for improving my design. An idea that I have includes removing RE for the CC stage and replacing with a current sink, but I am unsure. Also, maybe someone can point out issues with my current design.

Any help is appreciated. Thanks.

Also, I assumed beta is 150 for my calculations.

 

Hi,

Real quick, if your target gain is 50 and you already have that but can add one more transistor stage, then you might improve the linearity (and thus the THD) by using a third stage and dividing the gain up between those three stages instead of just two.

Also, you could simulate the circuit under LT Spice (if you havent already) and check the Fourier components and see if you can get the lower ones to go down a bit with maybe a 100Hz or 1kHz test signal.

 

Hi,

Real quick, if your target gain is 50 and you already have that but can add one more transistor stage, then you might improve the linearity (and thus the THD) by using a third stage and dividing the gain up between those three stages instead of just two.

Also, you could simulate the circuit under LT Spice (if you havent already) and check the Fourier components and see if you can get the lower ones to go down a bit with maybe a 100Hz or 1kHz test signal.

Hmm, ok. Thanks for the suggestion. However, what does "THD" stand for?

 

I think it means Total Harmonic Distortion. Didnt check with my dictionary.

 

Hi,

Yes the THD is a measure of how well the circuit reproduces the input. Low THD means it is good, high THD means it is not so good. With audio we like to get very low THD. With a servo amp it doesnt matter nearly as much.