Is the mean "Class AB" & "Push pull"

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haoyun

Joined: 10 Oct 2005
Posts: 81
Helped: 13

26 Sep 2007 14:45 Is the mean "Class AB" & "Push pull"

I think , a push-pull output stage can provide Vout from Vdd to GND. For example, Vdd=2V, Vdc=1V and Vss=GND=0V. In this situation, the output is push-pull but class A , not class AB, right?

Thanks a lot!

A.Anand Srinivasan

Joined: 15 Oct 2005
Posts: 1792
Helped: 216
Location: India

26 Sep 2007 16:10 Re: Is the mean "Class AB" & "Push pull&a

i push pull stage cannot give full output swing from Vdd to Gnd due to the voltage drop across due to the diode drops of the transistor and the resistors usage needed to avoid breakdown and good operation of the circuit...

haoyun

Joined: 10 Oct 2005
Posts: 81
Helped: 13

26 Sep 2007 16:52 Is the mean "Class AB" & "Push pull"

Yes,you are right. However, a push pull stage can give output swing from Vdd-Vosat to Vosat ( assume Vss=0). I want to know, this is class A or class AB?
Thanks your reply.

A.Anand Srinivasan

Joined: 15 Oct 2005
Posts: 1792
Helped: 216
Location: India

27 Sep 2007 1:00 Re: Is the mean "Class AB" & "Push pull&a

both can do the same and depends on the biasing....
I dont understand why you are talking about class A stage when you are clearly mentioning a push-pull stage....

haoyun

Joined: 10 Oct 2005
Posts: 81
Helped: 13

27 Sep 2007 9:50 Is the mean "Class AB" & "Push pull"

Class A and AB have diffrence power efficiency. I have to note this problem, because I want to design a low power OPAmp. I have another question , if Vss=0 and RL is limited value(for example 10KOhm), then at Vout=Vdd/2, iL#0 . -->This means, The IQ of this configuration is always denpendece on ZL(Load impendence)? Has any output configuration is independence on its load?

Thanks a lot.

A.Anand Srinivasan

Joined: 15 Oct 2005
Posts: 1792
Helped: 216
Location: India

27 Sep 2007 14:36 Re: Is the mean "Class AB" & "Push pull&a

any load would need current across it to satisfy the output voltage... also when it comes to low power design and power efficiency i think a class AB stage is better because there is no path between supply and ground unless a load is connected and this leads to significant reductionin power loss....