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the problem with the preamp

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ql396387901

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hello guys,

I have a problem with the small signal analysis with the preamp circuit. You can see the circuit in the picture. At the beginning I thought

the gain Av = 2*gm*Rd. But when I do the simulation, the result is difference with my analysis.

can you help me?




https://obrazki.elektroda.pl/85_1346598160.png
 

Are you counting the outputs individually? Or are you bridging across them?

Each output swings within a volt range which is the same swing as the input. This is because mosfets are voltage-controlled devices.

However notice that you can put a load across the outputs, and get 2x the voltage gain.

My simulation shows this amp can serve as a power amp. The topmost resistors can be as low as 10 ohms.
 

ql396387901 said:
At the beginning I thought the gain Av = 2*gm*Rd.
Click to expand...
It depends how you define gain.
If Vref1 = Vref2 = constant, and you define gain as: gain = (Vout+ - Vout-) / (Vin+ - Vin-), then gain = Rd*gm, where gm is the value for a single device.

BradtheRad said:
My simulation shows this amp can serve as a power amp. The topmost resistors can be as low as 10 ohms.
Click to expand...
With idling currents = 100uA? :-?
 

I wrote: My simulation shows this amp can serve as a power amp. The topmost resistors can be as low as 10 ohms.
Click to expand...

godfreyl said:
With idling currents = 100uA? :-?
Click to expand...

The 100 uA bias resulted in the output staying at the upper supply rail.

I found I had to increase the bias to several mA, just to bring the output level to a sensible area of operation.

For a while I tried high values for the upper resistors. However the outputs were mostly a straight line, except at input peaks.

I could not obtain linear response until I made the resistors around 330 ohms.
 

Hi,
if Vref+ and Vref- were tied together, then set them being equal to the common mode voltage of Vin+ and Vin-. the gain of the circut would be 2*gm*Rd, like simple differential pair.
The Cin(Cgs) could be halved at cost of twice power and addtional load cap. I don't think this circut work well as peramp, but it can be a good buffer. because it is difficult to get high gain and low noise.
Ziqi
 
Yes, I tend to agree with Ziqi, because my result (hand calculation) is as follows:

(Vout+)-(Vout-)=-2gm*[(Vin-)-(Vin+)+(Vref+)-(Vref-)].
 

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