How Do You Design a BJT Amplifier with Specific -3dB Points at 10Hz and 10kHz?

[Elektron]

I would like to know how to design an amplifier similar to the attached circuit with set -3db points, the lower on the order of 10Hz, and the higher on the order of 10kHz.

I have designed amplifiers to work with a set gain, but have not dealt with frequency response before.

I designed an amp on PSPICE with a theoretical gain of over 200, but when I tacked on a low-pass filter, the output pretty much went to 0.

Therefore, how would you go about designing for gain in addition to -3db points?

 

[berkeman]

Welcome to the PF.

Ignoring the emitter degeration, what RC time constants set the LPF and HPF breakpoints in that circuit?

EDIT -- Actually I'm not seeing the LPF part of this circuit...

 

[Baluncore]

In the simple analysis, gain is decided by the ratio of Rc to Re. But for a 10Hz to 10kHz band all sub-modules will effect each other to some extent.

Place a small value capacitor, Cbd, from node B to node D. That makes the BJT a miller integrator which is a low-pass filter. Select Cbd to give –3dB at 10kHz.

Both C1 and C2 set the 10Hz low frequency point. But their surrounding resistive circuit is difficult to analyse. Simulate it with C1 as a very high value capacitance, adjust C2 for –1.5dB at 10Hz. Remember that value of C2 for later. Then simulate it with C2 as a very high capacitance, adjust C1 for –1.5dB at 10Hz. Replace C2 with the value remembered earlier. The values of C1 and C2 that give –1.5dB each will together give –3dB at 10Hz.

Because the 10Hz HPF and the 10kHz LPF both give some in-band loss, you will need to adjust Rc/Re to increase the DC gain to compensate. Notice that Ce will give a step change to in-band gain if the value of Ce is too small.

 

[Elektron]

Thank you both for your replies. I was able to implement your suggestions on PSPICE and am observing the -3db points.

 

[alphy]

Designing an amplifier with specific -3db points can be achieved by incorporating a frequency-dependent feedback network into the circuit. This network can be in the form of a low-pass or high-pass filter, depending on the desired frequency response.

To design for a lower -3db point of 10Hz, you can use a low-pass filter with a cutoff frequency of 10Hz. This can be achieved by selecting appropriate resistor and capacitor values in the feedback network. Similarly, for the higher -3db point of 10kHz, a high-pass filter with a cutoff frequency of 10kHz can be used.

When designing for both gain and -3db points, it is important to consider the trade-off between gain and bandwidth. Increasing the gain of the amplifier will decrease the bandwidth, and vice versa. Therefore, it is important to carefully select the values for the feedback network to achieve the desired -3db points while maintaining the desired gain.

In your case, it seems that the low-pass filter you added to the circuit significantly reduced the gain, resulting in a near-zero output. This could be due to the fact that the cutoff frequency of the filter was too low, causing a significant attenuation of the input signal. By carefully selecting the cutoff frequency and component values, this issue can be avoided.

In summary, to design an amplifier with set -3db points, it is important to consider the frequency response of the circuit and incorporate a suitable feedback network to achieve the desired cutoff frequencies. This can be achieved by selecting appropriate resistor and capacitor values and considering the trade-off between gain and bandwidth.