Designing constant gain amplifier

[suria3]

constant gain amplifier

Hi guys,

I'm designing an amplifier (let say a differential amplfier). My target is to have a constant gain across temperature. Meaning if i have gain of 10dB at 27 deg, then at higher or lower temperature the gain should be about the same. Currently i have tried with the constant PTAT current method that to bias at the tail, but I'm not getting a constant gain, the gain does vary about 7dB. Please suggest me on how I can come out a constant gain amplifier.

Thanks,
Suria

 

[Fahmy]

What about using a diode-connected loads. The gain will be gm1/gm2, where gm1 is the tranconductance of the diff pair and gm2 is the transconductance of the diode-connected load. You can adjust the ratios of gm to get the 10dB gain. also you can easily match the two gms.

 

[suria3]

Will this diode connected configuration gives the constant gain across temperature. I knew the dfferential gain can be achieved by tuning the gm1/gm2 parameter, but the matter here is i can achieve a 10dB in a conventional resistive load amplfier but the constant gain is a matter. thx

 

[pixel]

I have made this with normal amplifier followed with VGA.
The key point is to design control loop with dummy amplifier and VGA that will vary VGA according to the temperature...you need bandgap..
Error is proportional to the mismatch between active and dummy structure...
so they have to be close...

 

[drDOC]

If you really want an accurate amplification you should use an feedback amplifier.

You can however make a sort of constant gm biasing.
What do you mean with constant PTAT current?
The transistors in your bias circuit should be operating at the same current density as the gm setting transistors. In such a way that the PTAT or PTIM (strong inversion) current compensates the gm change.
The voltage gain then depends on the load impedance of your diff pair. Maybe a thick film zero TC resistor can be used.
This way your gain can be constant over temperature, but it is not constant over corners and mismatch.

 

[gunturikishore]

Hi Mr Pixel,

Can you please elaborate where you will use that Bandgap circuit in the control loop??