# Design a BJT amplifier to meet all the given specifications (using LTspice)

Given parameters: 1 kHz, 10 mV sine wave as excitation (Vin).

Specifications to be matched:

Input resistance (Rin) = 18 kΩ,
Current gain = 33 dB,
3 dB bandwidth = 425 kHz

We are allowed to vary R2 and R3 (refer to 2nd figure below) and capacitor values to meet the specifications.

My attempt: I started with Rin and to match it, I varied R2 values using LTspice and got it to be 31.73 kΩ.

Now the problem is when I try to meet ideal current gain or 3 dB bandwidth specifications by fixing this R2 (= 31.73 kΩ) and by varying R3 or the capacitors, my Rin is bound to deviate from given specification I just matched, so I'm not really sure as to how to go forward and match other specification after the first one.

Fresh attempt: I took a different approach and only varied R2 and kept R3 fixed. I varied R2 values to match current gain of 33dB and got 46.22 kΩ and when I tried to match Rin, I got 29.19 kΩ, but then I realized that by changing the C2 value to say 706 nF the gap between the two R2 values gets bridged and I get 35.209 kΩ for matching current gain and 32.92 kΩ for matching Rin and 3 dB bandwidth isn't affected much and is nearly 416 kHz, but I do not have a concrete way to vary C2 (except randomly changing it) whereby I can simultaneously check which two R2 values gives required current gain and Rin and actually bridge the gap between the two.

EDIT: Model to be used:

As for the voltage gain spec, we aren't required to meet any, so we just have go as much closer to only the three given specs.

• I like how the 1st picture has two completely useless crossing dots at the collector, but none at the base. Mar 7, 2021 at 22:44
• This would be a job for an optimizer. Optimization can IIRC be scripted in LTspice, but there are tools that make it much easier. I'd suggest MicroCap. It's very good at showing when requirements are unreasonable and/or unfulfillable by only modifying a small number of components. Basically, showing whether this problem has a solution or not is a trivial job with MicroCap. I highly encourage you to give it a try. MicroCap has plenty of optimization examples. Jun 12, 2023 at 16:52
• Why are you using an obsolete BC107B transistor that is no longer manufactured? I used a few of them 62 years ago. Oct 12, 2023 at 19:04

Current gain of 33 dB means 44 and current GBW becomes ~15 MHz which is easily met.

But Rin depends on hFE * (Re+Rbe) which when bypassed for AC just becomes hFE * Rbe where Rbe = 26/Ic (mA) so that won't work.

No voltage gain spec? No Attenuation spec? your emitter cap is shorting the AC input at some frequency.

Output impedance is Rsource / hFE and 1mV is small and the current gain = hFE as long as load AC current does not exceed DC emitter bias current.

Try again.

• Thanks! Please have a look at the edits and the fresh attempt as well. Mar 8, 2021 at 8:55
• No. You have design that not only can be unstable with any circuit inductance, this is how not to design a signal amplifier. Why do you think this is a linear amplifier for the signal? Mar 8, 2021 at 13:46
• The "amplifier" has no voltage gain and has no load. Dec 31, 2022 at 16:03