single transistor audio amp

[Sceadwian]

Acording to my ltspice model the 3902 has a static current gain at a base current of 1ma of about 220

 

[audioguru]

where did you get 0.026 from?

0.026 is a basic number to calculate the internal emitter resistance of a transistor.

If the transistor doesn't have negative feedback to its base that interferes with the input signal, then its voltage gain is simply RC/(RE + Re). Re is the internal emitter resistance which is 0.026/IE.

The input impedance of a transistor is beta x (RE + Re). The input impedance graph of a 2N3904 on its datasheet disagrees somewhat.

 

[Roff]

That makes more sense now but where did you get 0.026 from?

Adding to what Audioguru said, Re=.026/Ie comes from the diode equation (Google).
I copied the following from one of my previous posts.

From the diode equation,

I = Is*(e^(qV/kT)-1) where Is is the reverse saturation (leakage) current

I/Is = e^(qV/kT)-1
I/Is ~ e^(qV/kT) for I>>Is

qV/kT = ln(I/Is)

V = (kT/q)*ln(I/Is) = (kT/q)*ln(I) - (kT/q)*ln(Is)

dynamic resistance = dV/dI

dV/dI = (kT/q)/I ; k is Boltzmann's constant, q is the charge on an electron in Coulombs, T is temperature in degrees Kelvin

kT/q = .026 at T=300 Kelvin

dV/dI=.026/I at T=300 Kelvin

The number I remember is (26 ohms)/(current in milliamps)

 

[mamun2a]

i questioned like a jack ass,
i wish for use a trns before spkr to doubling or rise the sound , here i c a lot of science to do so, i'm just illiterate to this.

 

[eng1]

That circuit is a shunt-shunt amplifier and I expect that the low cut-off frequency is quite high for an audio amplifier (unless you increase the input coupling capacitance). The input impedance is low because the feedback decreases it. It is useful when you want a low input impedance.
The closed loop gain is a 'trans-resistance' and it depends on the current gain of the transistor.

 

[Hero999]

Thanks for the help.

I suppose the only way to get a gain of anywhere near 4.7 is to use a darlington transistor like the MPSA14. One advantage of this circuit is the output impedance is much lower than Rc which isn't the case with a normal common collector amplifier.

 

[audioguru]

Hi Mamun,
The volume control on an amplifier is used to "double or rise" the sound.
The volume will increase if the power amplifier has enough extra power for it and if the speaker can operate with the extra power without damage. If the power amplifier doesn't have enough extra power to increase the volume then it will cause severe distortion.

A power amplifier would need to be completely re-built to increase its max output power.

 

[Cabwood]

Does this little amp need such a low output impedance? Why don't we just bump up RL? The current in the collector at the moment is so large that our feedback resistor cannot hope to source enough current to the base. OK, you could up the beta with a darlington pair, but I think a collector resistor of 4k7 should improve the gain a lot.

 

[audioguru]

I changed the collector resistor from 470 ohms to 4700 ohms. The DC operating point of the collector changed but the gain didn't change much. It was about 3.3 and it increased to only anout 4.5.

 

[Roff]

Does this little amp need such a low output impedance? Why don't we just bump up RL? The current in the collector at the moment is so large that our feedback resistor cannot hope to source enough current to the base. OK, you could up the beta with a darlington pair, but I think a collector resistor of 4k7 should improve the gain a lot.

Actually, with a beta of 100, the ratio of Rfb/RL=100 sets the collector voltage at approximately (Vcc+Vbe)/2, which is close to ideal. With higher beta, this voltage will be lower.

 

[Roff]

I changed the collector resistor from 470 ohms to 4700 ohms. The DC operating point of the collector changed but the gain didn't change much. It was about 3.3 and it increased to only anout 4.5.

Yeah, and look at the new DC operating point.

 

[Cabwood]

I changed the collector resistor from 470 ohms to 4700 ohms. The DC operating point of the collector changed but the gain didn't change much. It was about 3.3 and it increased to only anout 4.5.

Since the theoretical voltage gain of this amp is 47k/10k (for very high transistor beta), 4.5 is a much better performance than 3.3. 30% better. I wouldn't call going from 3.3 to 4.5 "not changing much". I'd rather call it "just about right".

And let me guess - the operating point was closer to 0.7V? For a better operating point (about half the supply), insert a 7k resistor from base to ground. That should do the trick, but I am guessing. And I'm also guessing that this is not a very linear amplifier for large(ish) swings. I do like the simplicity of it though.

 

[Roff]

Since the theoretical voltage gain of this amp is 47k/10k (for very high transistor beta), 4.5 is a much better performance than 3.3. 30% better. I wouldn't call going from 3.3 to 4.5 "not changing much". I'd rather call it "just about right".

And let me guess - the operating point was closer to 0.7V?

I agree with you about the gain, but I wouldn't call 0.7V (~0.95V, according to Audioguru's waveform) a good operating point.

 

[audioguru]

The transistor with the 4700 ohm collector resistor was biased with a 4.5VDC collector operating point when an 8.2k resistor was connected from its base to ground. As usual with a common emitter transistor without a current source for its collector load, the top of the waveform became compressed at high levels and got as high as only 8V with the 9V supply.