Mic preamp, LTSpice

upand_at_them

Joined May 15, 2010
940
I made this microphone preamp in LTSpice (haven't used LTS much before) and I noticed that the output signal has a fatter top than bottom....Or maybe it's the bottom that is skinnier than the top.

Did I do something wrong in the design? Or is this just the nature of the simulation? The signal isn't near either power rail.

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Papabravo

Joined Feb 24, 2006
20,379
You want Vb to be in the middle of the supply range and don't bypass the collector resistor.
Try something like this:

Boost the input AC signal to get a bigger output. The .png picture was take from a previous run where the input was 10 mV P-P so the RMS values are not correct. They should read:

virms: RMS(v(vin))=0.00353148 FROM 0 TO 0.001
vorms: RMS(v(vout))=0.361432 FROM 0 TO 0.001
av: vorms/virms=102.346

The actual design procedure for a CE amplifier is a bit more complicated than is practical for a forum post. What does the online textbook for this site have to say?
Sorry. It doesn't look like there is much practical design help there.

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Bordodynov

Joined May 20, 2015
3,087
With an input voltage source signal with an amplitude of 10mV the distortion is about 10%. The main harmonic distortion is second. And this is even before the voltage limits are reached. Do a Fourier analysis (.Four 2.5kHz).

crutschow

Joined Mar 14, 2008
32,852
Did I do something wrong in the design? Or is this just the nature of the simulation?
Not wrong just bad, and no.

And that distortion is not specifically related to the bias point.
You are seeing the inherent non-linearity of the large-signal Vbe voltage versus the collector current.
(The collector current is a logarithmic function of the base-emitter voltage.)
You can see that, since if you drive the amp with a current source instead of a voltage source, that distortion will be significantly reduced (as the collector current versus base current is fairly linear).

Alternately, if you add a small un-bypassed emitter degeneration resistor (R5, see below, which gives negative feedback) you can reduce that distortion with a voltage input.
The tradeoff is gain versus the reduction in distortion.
The higher the feedback resistor the lower the gain and distortion.

You can check the distortion using the .four command as shown by looking at the View/Spice Error Log.
Without R5 the harmonic distortion is over 8%.
With R5 the distortion is reduced to <3% (shown).

You can vary the value of R5 to see how the distortion changes.
To maintain the same DC bias conditions, the sum of R4 and R5 should stay constant.

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upand_at_them

Joined May 15, 2010
940
Great. Thanks guys! Looking forward to making changes and actually building it.

upand_at_them

Joined May 15, 2010
940
So this design is bad? What is bad about it? Is there a better design I should be looking at? I don't mind getting this one the best I can get it and then building a different one to compare.

upand_at_them

Joined May 15, 2010
940
I changed R3 to 1.2K, C3 to 220uF, and the new R5 to 20R. Distortion is now just under 1%.

upand_at_them

Joined May 15, 2010
940
You want Vb to be in the middle of the supply range and don't bypass the collector resistor.
I have the collector capacitor there because of someone else's design. It was said that it limits the bandwidth of the amp, since this is for a microphone and doesn't need beyond 20kHz. I haven't run the circuit through different frequencies yet, though.

crutschow

Joined Mar 14, 2008
32,852
I was referring to it not having any negative feedback to reduce the distortion.
I changed R3 to 1.2K, C3 to 220uF, and the new R5 to 20R. Distortion is now just under 1%.
Does it still have sufficient gain for your mic?

upand_at_them

Joined May 15, 2010
940
Does it still have sufficient gain for your mic?
Gain is less. But I figure that I can add a second stage.

Audioguru again

Joined Oct 21, 2019
6,152
An opamp has been used as a low distortion preamp for many years. Before that, a preamp with two transistors using lots of negative feedback for low distortion was used.

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upand_at_them

Joined May 15, 2010
940
I'm going to try an opamp after this. Thanks.

Audioguru again

Joined Oct 21, 2019
6,152
Do you have a common electret mic or an old dynamic (coil and magnet) mic?

upand_at_them

Joined May 15, 2010
940
Electret.

Audioguru again

Joined Oct 21, 2019
6,152
I have used this circuit many times. Texas Instruments shows an inverting opamp for a little less hiss noise.

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DickCappels

Joined Aug 21, 2008
10,067
Your original problem was caused by C2. I would leave that out.

upand_at_them

Joined May 15, 2010
940
@Audioguru again Thanks for the circuits. I do have some opamps around here somewhere.

@DickCappels I did test removing C2, though. Without it I get no noticeable change that I could see, but slightly more distortion (8.7% v. 8.4%). Even with my latest circuit changes I have 0.887% distortion with C2 and 0.996% without it. I don't know enough to comment on it, but if it has slightly less distortion and also limits bandwidth to 20kHz I'm in favor of it.

crutschow

Joined Mar 14, 2008
32,852
Your original problem was caused by C2.
No, the distortion was caused by the nonlinearity of the BJT as I stated in post #4.

Audioguru again

Joined Oct 21, 2019
6,152
A transistor with a high voltage gain (no negative feedback) and at a fairly high output level 9but not clipping) has severe even-order harmonics distortion. The top part of the waveform is squashed so much that the voltage gain cannot be measured.

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