question about symetrical swing

[walid1]

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?

 

[Kevin Weddle]

Approximately .5Vcc is the optimum collector voltage for maximum voltage change. A bipolar transistor can however drop as much as .3 volts, which means the collector voltage would need to be .3volts higher.

 

[walid1]

VC or VCE?

 

[Kevin Weddle]

A saturated bipolar transistor can have a voltage of .3volts. That is VCE. VC, or colector voltage would be .5Vcc + .3volts. But normally, the saturation voltage is near 0volts. In biploar transistor digital logic, the maximum logic low level is .8volts.

 

[audioguru2]

A transistor needs an emitter resistor to add some negative feedback so its output is not extremely distorted and non-symmetrical, and so that transistor Vbe differences and temperature changes do not mess up its biasing. Then the emitter resistor has voltage across it which reduces the output swing and requires the operating point to be higher.

 

[Kevin Weddle]

I'm sorry, that formula is not correct.  But the collector voltage is increased because of the saturated transistor voltage drop, if any, and the emitter resistor.

 

[audioguru2]

A single transistor with a high voltage gain produces an extremely distorted signal if its output approaches anywhere near cutoff because its transconductance is very non-linear.
Here is an example:

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