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Output stage in class A amplifier

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ee484

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hi, all.

If I have to design class A amplifier driving 4Ohm resistive load. Assuming the simple topology of class is bias current in the upper side and NMOS transistor in the lower side.

Say, I have output swing of 2V (output 3V process). Then, the voltage gain of the class A stage is roughly (assuming gm=20mS) 20mS*4 = 80mV/V. Then, the input of the gate should be 25V??

Waiting for your explanations.

Thanks,

B
 

about output stage

If you want an output swing of 2V for a 4ohm load and expect it to achieve with a driver of 20mS gm, then of course you must have a swing of 25V at the input to the driver.

Driving 4 ohm loads require power amps, which are capable of driving such a big load. These power amps have sizes in mm range and have gm much much greater than 20mS.
 

about output stage

I think to drive so large resistor load, class A output driver is not adaptive, at least class AB or class D.
 

Re: about output stage

u could do with typical low op impedance stage such as an opamp source folloer or
simple source follower .where u have op imp as 1/gm . u could use nmos at top and pmos at bottom config to have 1/2*gm op imp. latter is better as 40 is very less .
this class A u wont have good gain but very good attenuation as u have derived
 

about output stage

as Mists pointed out, Class A is mostly not used to drive large loads mainly from efficiency point of view. Either class AB or switching types like class D are used to get good efficiency. Whatever class is used, power amps are required that not only are capable of driving the loads but also able to handle the self heating of the devices.

Source follower may not be good for this application. You are trying to get a good swing out of the device. Source follower kills a bit of the swing and also, the previous stage has to swing by the same amount with a DC level vt above this stage. May be tough to achieve.
 

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