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Gain control in Amplifier

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Nigel Goodwin

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wait a minute , Feedback is a loop from output>feedback circuit>input>output , if we take emitter as output as you said , where is it connected to base ? i mean it is connected to base inside transistor, but where is the loop ??

Output is collector, input is emitter (as far as negative feedback is concerned).
 

MrAl

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wait a minute , Feedback is a loop from output>feedback circuit>input>output , if we take emitter as output as you said , where is it connected to base ? i mean it is connected to base inside transistor, but where is the loop ??

Everything is going to ground that is what is confusing from start :(

Hello,

Read post #18 which contains a full explanation.
 

neptune

Member
The two main goals of a circuit like this are to achieve some required level of AC gain while at the same time keeping the DC bias point as constant as possible with temperature. With low values of emitter resistance both the AC gain and the DC gain are high, but the DC bias point varies considerably. With somewhat higher values the DC bias point becomes more and more stable with temperature but we start to loose too much AC gain.
Choosing a value of emitter resistance that provides some AC gain and allow for some decent temperature stability means getting a lower than required AC gain but keeps the temperature stability within an acceptable range. Because we lost some AC gain in obtaining better temperature stability, we look for a way to increase the AC gain while not affecting the DC gain too much.
We know from experience that a capacitor passes AC current but mostly blocks DC current. By adding a bypass cap (or network) across the emitter resistor we get a higher Rc/Xe ratio for AC but maintain the same Rc/Re ratio for DC. This allows us to increase the AC gain while not affecting the DC gain too much and thus get more of the required AC gain while maintaining the desired temperature stability. (Note Xe here is an approximation of the emitter resistance in parallel with the emitter bypass capacitor reactance).
Damn wonderful idea !! but AC gain then would become frequency dependent.
if i only had temperature simulation in LTspice !
I will do simulation and post result soon ,
 
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ericgibbs

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hi,
LTspice does have a temperature Directive.

Gain of a single stage transistor amplifier with a emitter bypass capacitor is [AC] frequency dependent.
Look at the lower frequency roll off point on a 'AC Analysis' plot,
 

neptune

Member
hi,
LTspice does have a temperature Directive.

Gain of a single stage transistor amplifier with a emitter bypass capacitor is [AC] frequency dependent.
Look at the lower frequency roll off point on a 'AC Analysis' plot,
If i am wright Roll off is in 9.7Hz , where is temp. Directive here ?
 

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neptune

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audioguru

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Your transistor is biased wrong which is causing its output to clip. Its base voltage is too high which causes the emitter voltage to be too high. then its collector cannot swing lower than +4V. The value of the emitter resistor is also much too high.
Here is my simulation of your circuit:
 

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ericgibbs

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If i am wright Roll off is in 9.7Hz , where is temp. Directive here ?

hi,
Look at this simple amp, showing use of the Temp Directive.

EDIT:
Added your circuit, change the Analysis parameters and added Temp
 

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neptune

Member
Your transistor is biased wrong which is causing its output to clip. Its base voltage is too high which causes the emitter voltage to be too high. then its collector cannot swing lower than +4V. The value of the emitter resistor is also much too high.
Here is my simulation of your circuit:
you have changed nothing, Except Trans setting ?
 

neptune

Member
hi,
Look at this simple amp, showing use of the Temp Directive.
wow , it shows multiple graphs, what are these multiple graphs showing ?
 

ericgibbs

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wow , it shows multiple graphs, what are these multiple graphs showing ?

You should be able to explain what the multiple plots represent.

What do you think.?

EDIT:

Hint: Look at this image
 

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audioguru

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In your last simulation, you show 20mV input but the text reads 40mV peak. I didn't read your text so I showed an input of 20mV peak on my simulation that was clipping a little.

Here is my simulation with a 40mV input and its output is clipping a lot because the transistor is not biased correctly:
 

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neptune

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Every BC547 transistor is different and so is there gain beta also,
should i take general value of beta =250 for this transistor ?
what is the best way to biase it, to test every transistor in lab for its gain ?
 

ericgibbs

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Every BC547 transistor is different and so is there gain beta also,
should i take general value of beta =250 for this transistor ?
what is the best way to biase it, to test every transistor in lab for its gain ?

hi,
As each transistor/s have different gains, its required that the circuit provide negative feedback to stabilise the stage gain.
Either an emitter resistor or Collector to Base resistor.

EDIT.
An example, a single stage transistor amplifier using a BC547 with a Beta of 250, would be designed, using negative feedback, to give say a stage gain of 100.
Changing the BC547 for a Beta of say 200 would have only a minor change in the stage gain of 100.
 
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neptune

Member
Changing the BC547 for a Beta of say 200 would have only a minor change in the stage gain of 100.
how do we know what beta to take of transistor
 

audioguru

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You can buy a BC547A with low beta, a BC547B with medium beta or a BC547C with high beta. The manufacturer has tested them and sorted them into these 3 beta ranges. You can also buy an ordinary BC547 with a very wide range of beta.
The datasheet shows the range of beta at different currents and shows "typical" ones.

For learning, sketch a transistor with no emitter resistor then bias its base using "typical" beta. Then see that it is cutoff if its beta is low and that it is saturated if its beta is high.
Then add an emitter resistor and see that its DC operating point changes only a little when the beta is changed.
 
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