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Design >> Analog Design >> How to give feedback to a class AB push pull amplifier
https://designers-guide.org/forum/YaBB.pl?num=1320039085

Message started by mixed_signal on Oct 30^th, 2011, 10:31pm

Title: How to give feedback to a class AB push pull amplifier
Post by mixed_signal on Oct 30^th, 2011, 10:31pm

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

Title: Re: How to give feedback to a class AB push pull amplifier
Post by mixed_signal on Oct 30^th, 2011, 11:43pm

Is there any significance of input common mode range and output common mode range? I think input range and output range has significance since it is single ended

Title: Re: How to give feedback to a class AB push pull amplifier
Post by raja.cedt on Oct 31^st, 2011, 1:42pm

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.

Title: Re: How to give feedback to a class AB push pull amplifier
Post by loose-electron on Oct 31^st, 2011, 3:34pm

AC couple the input.
Put 1-10K ohms from output to input.

Title: Re: How to give feedback to a class AB push pull amplifier
Post by mixed_signal on Nov 3^rd, 2011, 12:23pm

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

Title: Re: How to give feedback to a class AB push pull amplifier
Post by raja.cedt on Nov 3^rd, 2011, 3:46pm

hello,
i didn't understand why u r getting such a low gain...please check where u r biasing, it should be the the place where u have more gain. I think 10k is fine (as long as it is more than r0)

Thanks,
Raj.

Title: Re: How to give feedback to a class AB push pull amplifier
Post by loose-electron on Nov 3^rd, 2011, 6:27pm

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.

Title: Re: How to give feedback to a class AB push pull amplifier
Post by RobG on Nov 3^rd, 2011, 7:02pm

I don't understand what you are doing if you are getting 0dB gain, but if you want to look at this circuit like an opamp consider the gate of Tn the "-" terminal, the source of Tn the "+" terminal, and the output would be the drain. The offset that you inquired about would be the gate-source voltage of Tn.

The DC operating point will be more stable if you replace Tp with a current source. As it is drawn, the DC current will depend strongly on the power supply.

Title: Re: How to give feedback to a class AB push pull amplifier
Post by loose-electron on Nov 5^th, 2011, 12:58pm

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

The DC operating point will be more stable if you replace Tp with a current source. As it is drawn, the DC current will depend strongly on the power supply.

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.