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Design >> Analog Design >> BJT operation
https://designers-guide.org/forum/YaBB.pl?num=1223456640

Message started by aaron_do on Oct 8^th, 2008, 2:04am

Title: BJT operation
Post by aaron_do on Oct 8^th, 2008, 2:04am

Hi all,

just wondering is there any reason why we absolutely should not bias the BJT in saturation for a small signal amplifier?

thanks,
Aaron

Title: Re: BJT operation
Post by vivkr on Oct 8^th, 2008, 2:28am

Hi Aaron,

The gain of the BJT drops, and also the recovery from saturation takes longer.

Regards,
Vivek

Title: Re: BJT operation
Post by aaron_do on Oct 8^th, 2008, 8:34pm

thanks.

The reason i'm asking is that I found that for my BJT, the fT is optimal when VBE is around 0.82. However, I cannot allow VCE>0.5 V so I'm worried that the BJT is in saturation mode. The gain is only slightly degraded under these conditions.

Aaron

Title: Re: BJT operation
Post by vivkr on Oct 8^th, 2008, 11:44pm

Hi Aaron,

The BJT is getting a bit into saturation mode with your choice of VBE and VCE although it is not yet deep in saturation if you are seeing reasonably high gain. In order to be sure that you really have reasonable gain, you need to first figure out how much lower you are allowed to get down with the VCE (accounting for variations in process, temperature, supply, and signal levels), and then also looking at the small-signal bandwidth in the worst case. If your VCE stays in the range of 0.4 V - 0.5 V, I would imagine that you might get away with it, but probably, your circuit will start to fail in terms of gain. I would check.

Regards,
Vivek

Title: Re: BJT operation
Post by ACWWong on Oct 9^th, 2008, 9:47am

VCE at >0.4V should be is fine, so long as you can tolerate the speed degradation.... I wouldn't consider device to be in saturation until 0.2~0.3V... obviously it depends on the junction areas but you can see the base-current shooting up when it becomes a problem.

Title: Re: BJT operation
Post by loose-electron on Oct 9^th, 2008, 12:00pm

Back to basics here guys -

1. Get a set of bias curves for the transistor.
2. Impose a load line on the bias curve set.
3. Impose an input signal onto the input and output of the load line and bias curve set.

That should answer your question if you are remaining within a desireable operating range. You are very much "inside the diode curve" but may still be within the "flat region" of the bias curve.

A very "1965" methodology to answer a question in 2008, but it works.

;)

Title: Re: BJT operation
Post by VINAY RAO on Oct 16^th, 2008, 11:26pm

Hi,
This is simple,if the device is operated in saturation then it cant amplify the signal,signal will be just clipped off.so to avoid clipping off u need to bias the device in active mode for BJT and saturation mode for mos.I hope this is clear... :)