Push Pull Opamps

any help regarding this :(

Aamir,

A push-pull output stage is one where the both the upper current sourcing transistor and the lower current sinking transistor can vary their current sourcing or sinking according to load conditions. The transistors could be N or P types, in either position, despending on the nature of the output stage design. Most all modern op amps have a push-pull output.

A rail-to-rail output is one that allows output voltage swing to be near the supply voltage. This can be accomplished in many different ways but almost always push-pull.

Miller effect can be generally defined as an altered impedance or gain due to feedback around a voltage amplifier. There are many on-line references and descriptions of this. The details depend greatly on the particular circuit involved. Your descritpion is not complete enough the provide much more insight. I suggest that you do some basic study of Miller effect and I think you will better understand how it applies to your circuit.

Regards, Bruce.

Aamir,

Push pull means the output current is bidirectional. It can source current into a load and sink current out of a load.

Rail to Rail output means the maximum output voltage is very close to the supply rails. These op amps have PNP or PMOS sourcing (top) transistors and NPN or NMOS (bottom) sinking transistors.

The schamtic shown is push(Q19)-pull(Q20) but is NOT rail to rail output.

The miller effect involves a feedback capacitor (C2) from the output back to the input of a gain circuit 180 of of phase. This increases the effect on the capacitance.

Regards,
Ron Michallick

Aamir,

I have attached an example of how to derive the Miller Capacitance in a Common Source Amplifier. You will see this in many different circuits however the approach you take to find this so called Miller Capacitance is generally the same. After you go through this example think about two things. The first is that using this approach makes hand calculation much easier but the main point to take is that you can place a small capacitance as shown with Cgd and effectively have what looks to the input like a large capacitance, hence the name Miller Effect. This is useful analysis to know when looking at the frequency response of amplifiers. Hope this helps.

Does that mean push pull op-amps can be any  which has npn or pnp transistor at o/p to sink or source current  &  have resistor R9 & R10 as I have attached pic above but rail to rail opamps may based on push pull technique & they don't have resistors which will cause voltage to drop.

Other difference is in rail 2 rail o/p, input also very close to rails.

Thanks Chris, Ron & Bruce 4 ur reply.

Aamir,

The diagram you posted is one type of push-pull design but it is not rail-to-rail. It is not R9 and R10 that poses the output voltage limitation, though they may have a small voltage drop. The output voltage of this design is primarily limited by the base-emitter voltage of the conducting output transistor. At best, this design can swing no closer than one Vbe from each rail. A rail-to-rail design would use a PNP for Q19 and NPN for Q20 with the collectors of both transistors connected to the output. Likewise, a CMOS R/R op amp would have the drains of the output transistors connected to the output.

Regards, Bruce.

Bruce Trump said:

The diagram you posted is one type of push-pull design but it is not rail-to-rail. It is not R9 and R10 that poses the output voltage limitation, though they may have a small voltage drop. The output voltage of this design is primarily limited by the base-emitter voltage of the conducting output transistor.

Vbe of transistor is 0.7V., isn't it after which it starts conducting right.

Yes, Vbe is approximately 0.7V.