Internal Input Bias Current Cancellation

RE: Internal Input Bias Current Cancellation

Sreehari kamsala — Wed, 10 Oct 2018 11:33:56 GMT

Hello Sir,

For Op Amps made of bjt the input bias current cancellation circuitry is resistive, what about the Op amps made of CMOS. I'm designing a simple two stage Op amp in which the bias current is comparatively high ( nearly 100pA ). Give me some inputs to reduce the input bias current further in CMOS Op amp.

Kindly suggest me some way sir,

Thanks in advance.

Sreehari

RE: Internal Input Bias Current Cancellation

Bruce Trump — Fri, 20 Jul 2012 18:04:28 GMT

Hi Ian. You may have a good reason to use a bias current cancellation resistor. I, of course, did not intend to rule out their use and I think you understand that. This comment area is not well suited to handle your question because you can't attach a schematic. I encourage you to post a question in the high speed amplifier forum. Though I don't normally troll the high speed amp forum, I will look for your post and comment. Be sure to include a schematic.

RE: Internal Input Bias Current Cancellation

Ian de Vries — Fri, 20 Jul 2012 17:38:45 GMT

Hello Bruce

So I have an application that needs high bandwidth >80Mhz and slew rate >40V/us and rail to rail output swing Vs = 12V.

Input offset voltage ideally as low as possible

The amps I have found have have input offset of around 100uV, but an Input bias current of 5uA. With a gain of 5 this voltage offset will give me a output voltage offset of around 500uV.

If I use a 2k input resistor, 5uA of bias current will give me a roughly 10mV offset at the output - considerably more than the voltage offset.

Hence in this case i think it is a good idea to use the cancellation resistors ?

your comments appreciated

thanks

Ian