MOSFET or NPN for class A output stage

There are a lot of pro's and con's for each device. It's impossible to say which is better, because no device is ideal.

MOSFET advantages:

Very high input resistance
No second breakdown

MOSFET disadvantages:

low gm
high and nonlinear input capacitance (HEXFET & co)
sensitive to parasitic oscillations


BJT advantages:

high gm

BJT disadvantages:

low input impedance
second breakdown effects
base currents
 

kASD

Member
2004-12-25 12:24 pm
N/A
amplifierguru said:
Not to mention that the Pd losses in a MOSFET output stage are considerably greater than a BJT stage - at least for emitter followers! Even greater waste of energy.

If the output swing is Rail to Rail and the output mosfets are only N-channel Vertical ones than NO BJT ON THIS PLANET DARES TO TOUCH THE EFFICIENCY CRITERIA OF N_CHANNEL VERTICAL MOSFETS...IN CLASS-AB AMPS

Greg,
I donot ..not at all agree with you......at this time.....

K a n w a r
 

kASD

Member
2004-12-25 12:24 pm
N/A
darkfenriz said:
Kanwar
Greg considered emitter (source) follower. Do you know any NCHMOS P-P amp with follower output (low open loop impedance)?


In N-Mos PP upper rail mos act as source follower amplifier and lower rail mos act as common source amplifier but it can be made to achieve Av=1 using "current gain cell" which qsc uses....
 
Hi Kanwar,

As Darkfenriz noted I was talking of EF output stage, so I agree with you that N channel with booitstrapped upper EF and CE lower half is R2R efficient. So is complementary CE output stage and with voltage gain in the output stage can have a higher slewing rate from same driver current as I = CdV/dt.

I have yet to see a simple N channel 'gain cell' that does not aspire to the dreaded cross conduction - especially those humongous monsters you use Kanwar!

Cheers,
Greg
 

kASD

Member
2004-12-25 12:24 pm
N/A
amplifierguru said:
Hi Kanwar,

As Darkfenriz noted I was talking of EF output stage, so I agree with you that N channel with booitstrapped upper EF and CE lower half is R2R efficient. So is complementary CE output stage and with voltage gain in the output stage can have a higher slewing rate from same driver current as I = CdV/dt.

I have yet to see a simple N channel 'gain cell' that does not aspire to the dreaded cross conduction - especially those humongous monsters you use Kanwar!

Cheers,
Greg


I have never ever constructed the gain cell in reality..oh yes i have tried it on the simulators....
regarding monsters i use my own customed designed pp drivers with floating power supply of 15voltsdc regulated to quench the thrist of huge gate capacitances......

cheers to you too.....
K a n w a r
 
Workhorse said:



In N-Mos PP upper rail mos act as source follower amplifier and lower rail mos act as common source amplifier
.........

hm.. a term 'follower' might be understood in a different way?

I thought upper NMOS works as current source because previous stage basically adjusts G-S voltage (same as lower) making the amp work like 'transconductance' one.
Follower in contrary is (for me) driving gate from fixed voltage (set to be adjusted comparing to ground, not output) and huge source local feedback.
Voltage source (follower) with current source (common source) in class (A)B simply doesn't make sense, what if I and V have phase difference of 40deg. ?

regards