Electronic – Input & output resistance in voltage amplifier design

amplifieranalogdesign

I'm currenly taking a course in Analog Circuits in the university and we got a task that requires us to design an amplifier, with characteristics that depend on our ID numbers. My amp should be a voltage amplifier, with the following requirements:

  • A = 59 dB
  • f_3db_low = 10 kHz, f_3db_high = 1 MHz (BW is approx. 1 MHz)
  • slope of rising (in Bode) = 40dB/Dec
  • slope of falling (in Bode) = -20dB/Dec

and in the frequency range 0.1 kHz < f < 100 MHz, the input & output resistances should be: R_in = 54 kOhm, R_out = 59 Ohm.

(Due to large R_in and small R_out, the amp is voltage amplifier)

The following circuit I designed satisfies all the conditions are satisfied, except the input & output impedances. I tried a lot of things, but it seems that everything I try just destroys the amplification, which is perfect to me.

I'm attaching a schematic of the circuit, Bode plot of the gain, R_in as a function of f, and R_out as a function of f (in this order):

The circuit:

Gain plot:

R_in:

R_out:

the circuit is based on CE, CE, CC stages with current mirror to each stage.

enter image description here

Best Answer

The input impedance is frequency dependent because of C4 in your circuit. Add an additional stage on the input without a capacitor in its emitter circuit.

R10 & R11 in your circuit have a bearing on the output impedance and they are too high in value. Reduce these values but this will reduce the gain as it will reduce the input impedance of the last stage. Find some replacement gain from the added extra input stage. R12 is not needed in your circuit.

The circuit below may not be perfect (say on output impedance, I don’t want to do your homework for you) but it’ll give you a good idea of what’s required as a progression. Try adjusting the values of various components and see how it affects the simulations to improve learning.

Transistor Amp

Transistor Amp

Set-up procedure would be (in order):

1) Set the I/P impedance by adjusting R5.

2) Set the O/P impedance by adjusting R9 & R10.

3) Set the Gain by adjusting R15 & R16.

4) Finally set the cut off frequencies by adjusting relevant capacitors.

This would be worth doing from a learning point of view and would give you an alternative design.

Related Topic