Hi,

I have a class AB push pull amplifier shown below. It has single ended input and single ended output.

1.How can I give a resistive feedback and use it.

2.How can i measure input common mode range and output common mode range?

3. How can I measure offset voltage? Is there any significannce of offset voltage in case of single ended input and output?

Thanks

hello,
why you want to use feedback here,if it is for input biasing better connect a resister from i/p to o/p. Once you have this big FB resister then offset is not imp. Common mode voltage is relavent for any case, it's significance is indipendent of single ended or fully diff.

thanks,
raj.

Hi,

1. How should i select coupling capacitors value?

2. I would like to remind that the circuit operates in subthreshold.

I AC coupled the input and connected 10K resistor between I/P and O/p. But the gain drops considerably below 0dB. I tried to minmise the feedback factor to 10 and even less. I tried various combinations with resistors and capacitors of different value but the gain drops.

I want feedback because it has a steep voltage transfer curve. By providing feedback I can operate at the high gain region without by output jumping  up and down. Moreover i want to do transient simulation to find the characteristics like i/p range, o/p range

Thanks

Tytpically that circuit does not operate subthreshold.

Quick approach to getting it running:

Connect the 10K resistor in to bias the device.

Increase the W/L of the devices unti you get a static current on the order of 100uA (its a starting point)

Keep W/L of PMOS 2X bigger than NMOS (better conductance in NMOS for same size)

AC couple the input signal, use a capacitor that has a Zw much less than the bias resistor. Start with 1uf and you can adjust that later.

Put the input signal on it, and run a transient simulation. Start with an input signal of 1MHz.

All of that is very simplified, but that should get you something with a gain of at least 4, and probably more.

RobG wrote on Nov 3^rd, 2011, 7:02pm:


Totally agree -
In a big way, it is two diode connected devices essentially.

However, these things do work pretty well, so lets get them started.

When it gets more sophisticated you put a control system around it to adjust the power supply voltage and maintain an average current, but thats getting ahead of things.