input signal amplitude

[amriths04]

dear friends,
in order to operate a common emitter as an amplifier in the linear region, is it necessary that we need to limit the input signal swing to about 20mv peak to peak?
why?

thank you in advance,

 

[tyassin]

Hi

Basicly the transistor is a non-linear device, you can see this on its Ic-Vbe characteristic.

If we are talking small-signal amplifier, then it is necessary to have a small voltage. Otherwise the amplifier will not be in a linear region. You can see this by super imposing a small sine-wave on the Ic-Vbe chracteristic around the bias of 0.7 V. If the signal is small, the curve will look almost linear. If you use a larger voltage swing the curve becomes non-linear.

There is a kind of rule of thump to have the signal voltage much smaller than the thermal voltage Vt:

Vin<<Vt

Hope this help

 

[dipal_z]

dear friends,
in order to operate a common emitter as an amplifier in the linear region, is it necessary that we need to limit the input signal swing to about 20mv peak to peak?

Yes but it will not be 20mv always it depends on gain and supply voltage of your amplifier.

why?

Because if your output voltage(which is input voltage multiplied by gain) after amplification goes above maximum output voltage that can be achieved with your design, output signal will be clipped and it will not be exact replica of input signal.

 

[amriths04]

ya i can get all that. but my basic doubt is in the case of a double stage or a triple stage amplifier.

consider a double stage...
assume the initial input signal amplitude=4mv(biased well within the linear region). let q1(gain)=20v/v. thus the input to the 2nd transistor q2 will be 80 mv. then do you mean there will be a non-linear amplification of the 80mv signal by q2?

and if that is so, then can we never use a transistor for amplifying more than 20mv?

i am confused.

thank you,

Hi

Basicly the transistor is a non-linear device, you can see this on its Ic-Vbe characteristic.

If we are talking small-signal amplifier, then it is necessary to have a small voltage. Otherwise the amplifier will not be in a linear region. You can see this by super imposing a small sine-wave on the Ic-Vbe chracteristic around the bias of 0.7 V. If the signal is small, the curve will look almost linear. If you use a larger voltage swing the curve becomes non-linear.

There is a kind of rule of thump to have the signal voltage much smaller than the thermal voltage Vt:

Vin<<Vt

Hope this help

 

[tyassin]

Hi

When you begin to calculate small signal gains, often only the gain seems interesting. But you have to consider the dynamics of the amplifier/circuit. As "dipal_z" wrote you have to consider bias as well. If foreksample you have a commen emitter amplifier. The gain of this can be expressed as the ratio of the combined resistance in the collector over the combined resistance in the emitter. Then you might think you can just pick resistances to give a gain of a 1000 or something. But if you then apply a small signal then you will see that if will not amplify with a 1000 because you will properly exsperience clipping.

But basicly you have to keep track of every point in the circuit and make sure it can provide the necessary signal swing to avoid saturation or clipping. So if you have 80mV, you could exsperience nonlinear effects and also problems like clipping due to not enough romm for signal swing.