Common Emitter Amp - modelled vs actual outputs

Thread Starter

timbradley

Joined Dec 15, 2020
1
Hi All

So I modelled a simple CE amp in LTSpice (asc file attached) which indicated a gain in the input signal of about 4.5x.

I then breadboarded the exact same circuit (with an A50k pot on the output) and put a tone generator through it and only managed to achieve about 1.5x gain. Also tried a number of other transistors (PN100, BC549...) with the same result.

Note the transistor has a measured hfe of 354 (from a DMM with a transistor port), but the model in LTSpice is an unmodified BC547B (hfe about 295).

Is this discrepancy between modelled and real-world response normal? what can I do to push the signal amplification higher?

I've also attached my spreadsheet calculating the resistor requirements for biasing and capacitor selection - being very new to electronics, I'd be keen for some QA.

I'd be using these for audio circuitry - primarily guitar pedals, so input frequencies are likely to be between 30Hz-5kHz to allow for harmonics.

Thanks in advance!!
 

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Audioguru again

Joined Oct 21, 2019
2,374
This circuit's input impedance is much too low for a guitar pickup. Usually the pickup feeds 1M or more.
The low input impedance of this circuit will kill the level and high frequencies from the pickup.
When fed from a low impedance signal, the gain is increased with a bypass capacitor parallel to the emitter resistor.
 

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LvW

Joined Jun 13, 2013
1,160
I then breadboarded the exact same circuit (with an A50k pot on the output) and put a tone generator through it and only managed to achieve about 1.5x gain. Also tried a number of other transistors (PN100, BC549...) with the same result.

Note the transistor has a measured hfe of 354 (from a DMM with a transistor port), but the model in LTSpice is an unmodified BC547B (hfe about 295).
Is this discrepancy between modelled and real-world response normal? what can I do to push the signal amplification higher?
Please note that it is not the hfe value which determines the voltage gain.
It is the transconductance gm at a given operational point (dc current Ic) which is responsible for the gain.
(The BJT acts as a current source - driven by the voltage Vbe).
A higher hfe value (for a fixed operational point) allows a larger input resistance of the stage - that´s all!
 

LvW

Joined Jun 13, 2013
1,160
hi Tim,
This LTS sim demonstrates the point in Post #3 regarding the effect of BF on the circuit Gain.
The BF values are shown on the diagram.
Question: Is it really the BF value which influences the circuit gain?

I don`t think so - because in both cases the DC quiescent point (operational point) is not the same (same resistor values are used in both circuits).
So - you are comparing two transistor stages with different quiescent collector currents Ic (different transconductance gm).

For my opinion, a fair comparison of two different transistors in a circuit should be done for equal DC operational conditions only.
 

LvW

Joined Jun 13, 2013
1,160
Question: Is it really the BF value which influences the circuit gain?

I don`t think so - because in both cases the DC quiescent point (operational point) is not the same (same resistor values are used in both circuits).
So - you are comparing two transistor stages with different quiescent collector currents Ic (different transconductance gm).

For my opinion, a fair comparison of two different transistors in a circuit should be done for equal DC operational conditions only. For equal operating points, both transistors (both hfe (BF) values) will have the same voltage gain.
 

Audioguru again

Joined Oct 21, 2019
2,374
How did you get a voltage gain of only 1.5 times? Mine shows a voltage gain of 4.5 times when biased with lots of current and a voltage gain a little less at 4.125V when biased with minimum current without any clipping.

Notice that on my simulations I show the DC output to show if the collector current is high or is low, and how far from clipping is the output.
 

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ericgibbs

Joined Jan 29, 2010
11,577
hi agu,
If you are asking me.?

I just copied the TS's circuit as posted, no changes.
Used a BC547B_ from my standard.bjt and increased the BF to 350 for the right hand circuit on that sim.
The left side BC547 is the regular LTS transistor.

I would be interested if you can identify any differences.

E

BTW: Please post your asc file and I will give a run.

Update:
Clip from TS's opening post.
So I modelled a simple CE amp in LTSpice (asc file attached) which indicated a gain in the input signal of about 4.5x.

I then breadboarded the exact same circuit (with an A50k pot on the output) and put a tone generator through it and only managed to achieve about 1.5x gain. Also tried a number of other transistors (PN100, BC549...) with the same result.
 

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ericgibbs

Joined Jan 29, 2010
11,577
hi agu,
Using the TS's original circuit.

The transistor in the LTS sim is not close to cut off, there is a 2V overhead voltage between Vc and Ve on the low Vout swing.

Also no distortion on the sine wave output, created a Vref sine wave source,[ see image] it overlays exactly the transistor Vout.

I suspect it could be a problem in LTS or the modelling.

E
 

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Audioguru again

Joined Oct 21, 2019
2,374
hi agu,
Using the TS's original circuit.
The transistor in the LTS sim is not close to cut off, there is a 2V overhead voltage between Vc and Ve on the low Vout swing.
Also no distortion on the sine wave output, created a Vref sine wave source,[ see image] it overlays exactly the transistor Vout.
I suspect it could be a problem in LTS or the modelling.
E
Hi Eric,
Tim wanted more gain so I added an emitter resistor bypass capacitor to cause the gain to be 120 times (with lots of distortion).
 

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