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Design >> Analog Design >> single supply class AB amplifier
https://designers-guide.org/forum/YaBB.pl?num=1173732209

Message started by rivercam on Mar 12^th, 2007, 1:43pm

Title: single supply class AB amplifier
Post by rivercam on Mar 12^th, 2007, 1:43pm

Hi,
iam kind of novice to circuit design. I wish to design a push pull amplifier with single supply. I understand the push pull configuration for dual supply. I simply not getting the concept for single supply.

In single supply, at maximum my input voltage can be vdd--ground. In that case what does postive and negative cycle of input means? For example in a 3.3 v supply if i assume a sine wave is riding on VDD/2 as my input, then both of my transistors are in ON from VDD-VTHP to VTHN in one direction and vice versa in the other direction. There where is push pull action here? In other wrds i was unable to see the push and pull currents for negative and positive half cycles and what iam seeing is short circuit current from vdd- gnd when i probed at PMOS and NMOS transistors.

To be honest i dont know how to see the push pull currents. Am I needed to connect my load resistor from output to VDD/2 or to ground?

If some one can explainin me the transient action with sine wave as an input, i would greatly appreciate you.

If my question sounds silly,plz excuse me.

Thanks in advance

Title: Re: single supply class AB amplifier
Post by chase.ng on Mar 12^th, 2007, 8:42pm

VDD/2 is known as your signal ground so positive cycle swing above it, and negative cycle swing below it. 0 V is the supply ground. I guess you connect a resistive load to your supply ground therefore you didn't see the NMOS pulling. It will be different if you are using capacitve load or connect it to VDD/2. You should see the push pull current if the load is connected to VDD/2 (signal ground) but it might not be available in a real single supply circuit (otherwise it will be a dual supply circuit :) ). I am not sure about your application, but I would choose a class AB bridge configuration in order to drive a resistive load in single supply.

Title: Re: single supply class AB amplifier
Post by rivercam on Mar 13^th, 2007, 1:02pm

Hi Chase,
My thanks for your reply. I Understand your explianation of ac ground concept but still unable to see the pushpull currents after connecting end of resistor to VDD/2.

well, iam trying to simulate the circuit in spice engine. Exactly my circuit is an inverter with shunt feed back. My load resistor is connected between output node and VDD/2. COuld you please elobrate me how should i setup my simulation to probe the push and pull currents.

Also you mentioned, its different if the load is cap-- how does circuit work in this case. If you feel short of time to explain lease refer some document/notes which is with single supply.

Thanks in advance

Title: Re: single supply class AB amplifier
Post by chase.ng on Mar 14^th, 2007, 7:34am

If it is a capacitors, it will charge at positive cycle and discharge at negative cycle. So it push and pull current from the cap. It is hard to me to debug your simulation without actually seeing it. It would be good if you can post your schematic here. I am sure there are a lot fine gentlemen are willing to help also.

Chase.