MOD NOTE: Moved to Homework Help.

I moved this to Homework Help because it's pretty apparent that the purpose of you being given this task is not to end up with an amplifier that has a gain of 1000, but for you to learn something about designing multi-stage amplifiers. That goal is best served if you are guided, however painfully for everyone involved, from wherever your current understandings (and misunderstandings) are toward a solution.

 

Where is Ve?
What is the purpose of RB22?

You can see Ve in the bottom left, Vs is taken out of the collector of Q2. RB22 is here to bias the transistor Q2 ( limit the current in the base of Q2 )

 

Your signal source says it is 1 µA, even though it is called Ve, which implies it's a voltage.

Is that a typo?

If so, what is the scale of your input signal?

Where is your output voltage being taken? I'm guessing it's at the collector of Q2, but engineering is not about guessing -- make your intentions explicit.

 

You can see Ve in the bottom left, Vs is taken out of the collector of Q2. RB22 is here to bias the transistor Q2 ( limit the current in the base of Q2 )

How is it going to do that?

 

Your signal source says it is 1 µA, even though it is called Ve, which implies it's a voltage.

Is that a typo?

If so, what is the scale of your input signal?

Where is your output voltage being taken? I'm guessing it's at the collector of Q2, but engineering is not about guessing -- make your intentions explicit.

Yes, Ve could be expressed as Vin, and Vout is taken out of CQ2. Ve is 2uA pic to pic. On the simulations, i effectively have a gain of 1000 ( 2mA on the CQ2 ) the fact is i can't find the mathematic epxression of Vs ( Vout ) / Ve ( Vin )

 

How is it going to do that?

RB22 just control the voltage gain ( not decoupled by a condensator ) and will affect the polarization because it drain some current out of CQ1

 

Yes, Ve could be expressed as Vin, and Vout is taken out of CQ2. Ve is 2uA pic to pic. On the simulations, i effectively have a gain of 1000 ( 2mA on the CQ2 ) the fact is i can't find the mathematic epxression of Vs ( Vout ) / Ve ( Vin )

You are looking for a voltage gain, but 2 mA is NOT a voltage, it is a current.

Is your input signal a voltage, or is it a current?

Is your output signal a voltage, or is it a current?

If either is a current, you are not looking for a voltage gain. If one is a current and the other is a voltage, then you have either a transimpedance or a transconductance amplifier. If the output is a current, then you need to indicate how that current is being taken out of the amplifier -- it can't be open circuit. Each of these types of amplifiers is a very different critter.

 

You are looking for a voltage gain, but 2 mA is NOT a voltage, it is a current.

Is your input signal a voltage, or is it a current?

Is your output signal a voltage, or is it a current?

If either is a current, you are not looking for a voltage gain. If one is a current and the other is a voltage, then you have either a transimpedance or a transconductance amplifier. If the output is a current, then you need to indicate how that current is being taken out of the amplifier -- it can't be open circuit. Each of these types of amplifiers is a very different critter.

OMG i'm so ******, ofc it's a voltage, it's 2uV ! My bad... This circuit as been tested as a Voltage Amplifier of 1000, that's a fact. Now, i need to express by calculations the Voltage gain of this circuits, or either, find a more effecient way to do it. That's why i'm asking for help ( My bad for this mistake )

 

and will affect the polarization because it drain some current out of CQ1

If you mean signal polarity then no, it won't.

 

If you mean signal polarity then no, it won't.

Without it, i can't have Ic2 = beta*Ib2, i cheked it with simulations (maybe you have a better option, i'm here for that ! ). Either, it does affect the gain.

 

Without it, i can't have Ic2 = beta*Ib2, i cheked it with simulations (maybe you have a better option, i'm here for that ! ). Either, it does affect the gain.

How does having it allow you to use that equation?

Oh, it will affect the gain, but only indirectly. The first stage has a certain output impedance and RB22 acts as a load on it that is in parallel with the load presented by Q2. If the output impedance of the first stage is low enough, then RB22 has almost no effect on how much current is drawn by Q2.

You don't want to design circuits based on the beta of a transistor -- this is a very poorly characterized parameter of a transistor that varies significantly from one transistor to another, even from the same batch, and also varies enormously over different operating points of the same transistor, not to mention over temperature. If your design relies on a particular transistor beta to work. then it is a poor design.

 

How does having it allow you to use that equation?

Oh, it will affect the gain, but only indirectly. The first stage has a certain output impedance and RB22 acts as a load on it that is in parallel with the load presented by Q2. If the output impedance of the first stage is low enough, then RB22 has almost no effect on how much current is drawn by Q2.

You don't want to design circuits based on the beta of a transistor -- this is a very poorly characterized parameter of a transistor that varies significantly from one transistor to another, even from the same batch, and also varies enormously over different operating points of the same transistor, not to mention over temperature. If your design relies on a particular transistor beta to work. then it is a poor design.

Here, the exercice is to do a circuit, with 2 NPN 2N2222, and they should not be seperated by a condensator, with a voltage gain of 60dB at 1khz. Both should be in Linear mode, so it involve that Ic1 = beta*Ib1 and Ic2 = beta*Ib2 AND Vce1 > Vcesat , Vce2 > Vcesat . This circuit does work with severals references of NPN. I tested it, also on severals simulators. The fact is i can't find the mathematic expression of this voltage gain. I'm asking for that. Or either, another design. Here is an exemple :

 

Have you analyzed the DC operating point of that circuit yet?

That's the first step.

Make sure that the transistors are biased in the linear region before going on.

Then, have you determined the transconductance of the transistors at their operating point?

 

Have you analyzed the DC operating point of that circuit yet?

That's the first step.

Make sure that the transistors are biased in the linear region before going on.

Then, have you determined the transconductance of the transistors at their operating point?

Already done that, DC analysis goes pretty well, AC analysis is like... Something else, that's here where i'm maybe not good enough in maths. Here's what i did

 

The fact is i can't find the mathematic expression of this voltage gain.

There won't be a simple expression, because the impedances of the components around one transistor affect the performance of the other transistor.

 

The voltage to current gain (transconductance) of a BJT is Ic / Vt where Vt is the thermal voltage (≈26mV @ room temp).
That's the formula to use to calculate the gain.

 

The correct way to control the gain is to split the emitter resistor into two in series and bypass only one of them with the capacitor. Then the gain is approximately the ratio of the collector resistor to the non-bypassed emitter resistor.

 

There won't be a simple expression, because the impedances of the components around one transistor affect the performance of the other transistor.

Totally, this is why the design of this design is hard, and that's why i'm asking for help.

 

The voltage to current gain (transconductance) of a BJT is Ic / Vt where Vt is the thermal voltage (≈26mV @ room temp).
That's the formula to use to calculate the gain.

but that's a part of all the formula, the fact is i can't express Vout/Vin

 

The voltage to current gain (transconductance) of a BJT is Ic / Vt where Vt is the thermal voltage (≈26mV @ room temp).
That's the formula to use to calculate the gain.

Can you do a schema of it ? I don't really understand