Can someone please tell me what function C2 plays in my circuit ?

All i can see is that it gives additional gain, is there any other reason for it being there?

 

Ask yourself why Re is there and what effect it has on AC signal gain without C2.

 

Can someone please tell me what function C2 plays in my circuit ?

All i can see is that it gives additional gain, is there any other reason for it being there?

Without C2, what is the gain of the circuit?

How does making the value of RE smaller affect that gain?

What happens to the impedance of the RE. C2 combination when they are in parallel as C2 is made reasonably large?

 

I should stress i havn't actually built the circuit so i have no idea what the gain is...i was just curious about what that C2 actually does, i see lots of common emitter amplifiers using it.

 

It bypasses the emitter resistor to increase the AC gain. Otherwise emitter degeneration would reduce it to the DC gain level.

 

So do you know how the emitter impedance value affects the circuit gain, i.e. how is the gain value of that circuit is calculated?
Once you understand that, you can answer your own question.

 

I should stress i havn't actually built the circuit so i have no idea what the gain is...i was just curious about what that C2 actually does, i see lots of common emitter amplifiers using it.

Sounds to me you need to study a little basic transistor amplifier theory.
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-4/common-emitter-amplifier/
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-4/feedback/

 

So do you know how the emitter impedance value affects the circuit gain, i.e. how is the gain value of that circuit is calculated?
Once you understand that, you can answer your own question.

I have no idea, i am doing the suck it and see approach (put some values in and see what happens) i really don't have much math knowledge other than ohms law.

 

Sounds to me you need to study a little basic transistor amplifier theory.
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-4/common-emitter-amplifier/
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-4/feedback/

thank you for that direction, i will find time to read those links.

 

I have no idea, i am doing the suck it and see approach (put some values in and see what happens) i really don't have much math knowledge other than ohms law.

For that particular amplifier configuration, the gain is basically the ratio of Zc/Ze where Zc is the impedance between the collector and its supply and Ze is the reactance between the emitter and its supply.

You are limited by how much DC gain you can tolerate, otherwise the amplifier will saturate even with no input signal due to the bias network. That limit is often a lot less than the gain we want at the signal frequencies of interest. So we design the emitter impedance such that it starts of at a fairly large value at DC (chose to put the quiescent operating point where we want it, which is usually somewhere near the middle of the rails) but that it's starts getting smaller as the frequency increases.

That's what the bypass capacitor does. At DC, it looks like an open circuit, so the emitter impedance is just the resistor. But at high frequencies, the capacitor looks very close to a short, which still looks like a short when you put it in parallel with the resistor. The RC time constant determines the frequency at which this transition happens and to choose it to be somewhere between DC and the bottom of your signal frequency range.

 

Can someone please tell me what function C2 plays in my circuit ?

All i can see is that it gives additional gain, is there any other reason for it being there?

I am "afraid" that - if somebody really wants to understand the role of RE and C2 - he has to learn the basics of negative feedback (and how negative feedback determines the gain)

 

I am "afraid" that - if somebody really wants to understand the role of RE and C2 - he has to learn the basics of negative feedback (and how negative feedback determines the gain)

Really, it is basic circuit theory! During the negative slope of the output waveform, the emitter resistor is in series with the load. The "increase in gain" is because of the effective reduction of the emitter series impedance by the capacitor C2

 

For that particular amplifier configuration, the gain is basically the ratio of Zc/Ze where Zc is the impedance between the collector and its supply and Ze is the reactance between the emitter and its supply.

Because no parts values are given for the circuit under discussion, I think it is helpful (and necessary) to add that the mentioned approximate gain expression (Zc/Ze) is valid for |Zc|>>25mV/Ic only.
For C2=0, this condition simplyfies to Re>>25mV/Ic.

 

For starters, the drawing of the input and output voltage waveforms are drawn incorrectly.



The common emitter amplifier is an inverting amplifier. Hence the output waveform needs to be inverted as shown above.

 

In order to have a deeper understanding of how transistor circuits behave, start with a basic common emitter configuration and learn how the base bias voltage affects the DC operating point of the transistor. You can do this with real components or with a circuit simulator.

From this, you can determine the current gain of the circuit.

 

Next, apply a sine wave signal to the input and determine the AC voltage gain.

 

Try it again with a resistor on the emitter leg.

 

The question is: What is the purpose of the emitter resistor, R3?

R3 introduces negative feedback. With more positive input at the base, the current through Q1 increases. The voltage at the emitter rises and hence the base-emitter voltage is reduced. This opposes the attempt to increase the transistor current. The end result is lower AC gain.

Now what happens when you put a capacitor on the emitter? The capacitor tries to keep a constant voltage on the emitter. Hence the emitter is now shorted to GND from an AC perspective, restoring the higher AC voltage gain.

The value of the shorting capacitor is important. It has a time constant when R3 is taken into account. Hence the capacitor only comes into play when the AC frequency is above a certain frequency. You choose the value of the capacitor depending on where you want the transition from low AC voltage gain to higher gain.

The take away here is: the function of the emitter resistor R3 is more important than that of the bypass capacitor.

The emitter resistor:

1. introduces negative feedback
2. introduces DC stability
3. makes it less temperature dependent
4. improves linearity to AC voltages
5. makes the circuit less dependent on transistor specifications

The bypass capacitor allows you to restore the voltage gain above a given frequency.

 

The question is: What is the purpose of the emitter resistor, R3?

R3 introduces negative feedback. With more positive input at the base, the current through Q1 increases. The voltage at the emitter rises and hence the base-emitter voltage is reduced. This opposes the attempt to increase the transistor current. The end result is lower AC gain.

Now what happens when you put a capacitor on the emitter? The capacitor tries to keep a constant voltage on the emitter. Hence the emitter is now shorted to GND from an AC perspective, restoring the higher AC voltage gain.

The value of the shorting capacitor is important. It has a time constant when R3 is taken into account. Hence the capacitor only comes into play when the AC frequency is above a certain frequency. You choose the value of the capacitor depending on where you want the transition from low AC voltage gain to higher gain.

Thanks for that explanation MrChips...

 

The disadvantage of that circuit with the emitter capacitor bypass, is that the gain is temperature sensitive, and the output can become non-linear at the frequencies where the capacitor totally bypasses the AC signal.
To stabilize the high-frequency gain and reduce distortion a resistor can be added in series with the capacitor.
The high-frequency gain is then mainly determined by the relative values of that resistor in parallel with the bias emitter resistor and the collector resistor.