BJT amplifier design

Question

I am trying to design a BJT amplifier with the given parameters and limitations. The circuit is shown below.

Parameters:

• Vcc = 15 V
• RL = 20 kΩ
• beta = 180
• Vbe(on)= 0.6 V
• C_µ = C_π = 0

Limitations are:

1. Swing voltage of V_out must be between 22 and 24 V (peak to peak)
2. Voltage gain must be more than 10
3. Power dissipation by resistors must be under 0.2 W
4. Corner frequencies of C₁, C₂, and C_E should be < 10 Hz
5. The high threshold frequency should be 20 kHz

I assumed that Rc must be smaller than RL so I designed Rc= 2 k ohms and then I found RE1 = 180 ohms, but I have an issue where I have to design RE2. I don't know how to use 22-24 V pk-pk specification in this design.

• How do I adjust peak to peak voltage?
• How can I calculate input and output impedances and open circuit voltage gain?
• How do I approach power dissipation considerations?

As it's a homework question, I'm not looking for a whole solution, but some tips and advice.

Answer 1

Just some of my initial thoughts. Perhaps they may help:

• Your specifications appear to hold a fixed \$V_{_\text{BE}}\$ value. This never happens, in practice. In fact, \$V_{_\text{BE}}\$ is continually changing with the applied signal and will distort the output and complicate (somewhat) the situation with such a tight peak-to-peak limitation range. But as luck would have it, you don't need to care about this reality. Nice. This means that you don't have to deal with \$r_e^{\,'}\$ as there's no dynamic resistance, which leads into the next note:
• For purposes of arranging the specified loaded voltage gain of \$A_v=10\$, you treat the collector resistor as being in parallel with the load. So, this means your goal is to have \$\frac{R_{_\text{C}}\,\mid\mid\,R_{_\text{L}}}{R_{_{\text{E}_{1}}}}=A_v\$.
• I'd select \$23\:\text{V}_{_\text{PP}}\$ (roughly) to allow some margin and I'd constrain the loaded gain to just slightly more than the allowed 10. So the required input will be \$2\frac14\:\text{V}_{_\text{PP}}\$ and the loaded voltage gain will be taken as \$A_v\approx 10.2\$.
• \$C_{_\text{E}}\$ will be taken to be large enough that its voltage remains fixed.

The power constraint may prevent success. The smaller you make \$R_{_\text{C}}\$ the closer the collector swing is to the specified swing on the load and this helps better fit it within the rails. But it increases the quiescent current required. And that is something that cannot be easily afforded. And if you focus on keeping the power manageable by reducing the quiescent current and increasing \$R_{_\text{C}}\$, the circuit must handle an ever-increasing collector swing. And that is also something that cannot be easily afforded. I've not spent enough time on this and I may have made a mistake, but a back-of-envelope calc suggests some worry.