# Common Emitter Amp - modelled vs actual outputs

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.

#### Attachments

• 77 KB Views: 18
• 1.6 KB Views: 32

#### 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.

#### Attachments

• 44.7 KB Views: 61

#### 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!

#### ericgibbs

Joined Jan 29, 2010
11,577
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.

E

#### Attachments

• 36.8 KB Views: 43

#### 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 dont 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 dont 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.

#### Attachments

• 43.9 KB Views: 26

#### ericgibbs

Joined Jan 29, 2010
11,577
hi agu,

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.

#### Attachments

• 3.1 KB Views: 3

#### Ylli

Joined Nov 13, 2015
999
(with an A50k pot on the output)
Are you sure that is a 50K pot? What load is on the output of the pot? Gain falls to about 1.5 with a 240 ohm load.

#### Audioguru again

Joined Oct 21, 2019
2,374
Hi Eric,
When a transistor is close to cutoff then its voltage gain is reduced, the top of the waveform appears squashed.

#### 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

#### Attachments

• 46.6 KB Views: 7

#### 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).

#### Attachments

• 63.5 KB Views: 10