question about symetrical swing

[nader55]

Hi
when i want to design a preamplifier using BJT and need to have a maximum symetrical voltage swing at the output, then two conditions must be satisfied:
1- VCE = 0.5 VCC
2- I (saturation) = 2 IC

Is there other conditions must be taken into acount?

 

[ericgibbs]

Hi
when i want to design a preamplifier using BJT and need to have a maximum symetrical voltage swing at the output, then two conditions must be satisfied:
1- VCE = 0.5 VCC
2- I (saturation) = 2 IC

Is there other conditions must be taken into acount?

hi,
If your Class A pre amp has an emitter resistor, you should allow for the voltage drop across it reducing the Vcc/2 point, also the load impedance the preamp is driving into as the will load the collector output. The collector resistor and driven load resistor in parallel.

What signal do plan to pre-amp.?

 

[nader55]

hi,
If your Class A pre amp has an emitter resistor, you should allow for the voltage drop across it reducing the Vcc/2 point, also the load impedance the preamp is driving into as the will load the collector output. The collector resistor and driven load resistor in parallel.

What signal do plan to pre-amp.?

Thank you ericgibbs very much, I plan to amplify a voice signal
my batt voltage (VCC) = 9 v, BJT is 2N3904
I choose IC = 1 mA
yes my Class A pre amp has an emitter resistor.
thank you a gain

 

[audioguru]

A single NPN transistor with a voltage signal input and a lot of AC gain is extremely distorted when its output tries to swing up to its positive supply voltage. So it will not be symmetrical like from an opamp.

Here is the simulation of a 2N3904 low distortion transistor with an AC voltage gain of 160 and biasing for a symmetrical output swing. Its output tries to swing up to +9.5V but can't and makes it up to only +8.5V instead with horrible distortion. Changing the biasing won't make much difference.

 

[gilmanli]

Hi audioguru,
I am a new using LTspice to do the simulation,how do you do the audio distortion test using LTspice,would you mind describe detail here
?

Best regard,
Gilman Li.

 

[nader55]

Thank you all, but that was my question"

Hi
when i want to design a preamplifier using BJT and need to have a maximum symetrical voltage swing at the output, then two conditions must be satisfied:
1- VCE = 0.5 VCC
2- I (saturation) = 2 IC

Is there other conditions must be taken into acount?

 

[audioguru]

I can see the distortion and hear it on sine-waves. the top section of the sine-wave in my simulation is badly compressed because the transconductance of a transistor is not linear.
There is a way of seeing the harmonics of a sine-wave in LTspice but I have never done it. Yahoo has a users group.

I normally use opamps for audio. The opamps have distortion so low that you cannot see it nor hear it and almost not measure it.

 

[gilmanli]

Understand,thank Audioguru.

 

[ericgibbs]

Understand,thank Audioguru.

hi,
For reference, this is the FFT of 'agu's circuit , showing the harmonics in the output due to the distortion, using LTspice.

 

[audioguru]

The transistor has a lot of even-harmonics just like a vacuum tube. If transistors are connected push-pull then most of the even harmonics cancel.