Author Topic: How to bias a common emitter amplifier without voltage divider network?  (Read 1113 times)

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Offline DmeadsTopic starter

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Hello All!

In My Analog IC design class, we are learning about active loads with current mirrors. I want to simulate some of these in Ltspice.

I am first working on a common emitter amplifier. I am having trouble setting the Vbe voltage to 0.7 volts. I read somewhere that you can still use the 4 resistor biasing network to set a base voltage, but the collector resistor can be an active load/current source. This still has 2 resistors tho (if the emitter resistor is a short).

I know that in IC design, it is hard to build resistors onto a chip because they take up space, so I'm wondering if anyone knows any ways that involve an active load to set a base voltage/operating point for a common emitter amplifier? Or is this a case in IC design where you just have to use resistors?

any .asc files would be appreciated!

Thanks

 

Online Zero999

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The simple common emitter configuration is seldom used in ICs and certainly not in isolation. The problem is, the transconductance, therefore voltage gain, is so high, it's not possible to set VBE, without the transistor either being in cut-off or saturation.

Most amplifier ICs have a differential pair at the input, essentially an op-amp. Quite often, as in the case with voltage regulators, it's buried with with inputs connected to a reference and potential divider.

Awhile ago, I built a discrete op-amp, for educational purposes. Here's the LTSpice file. Note there is a common emitter amplifier in there, Q9, but it's within the feedback loop of the differential pair, which sorts the biasing out, with negative feedback.

https://www.eevblog.com/forum/beginners/how-hard-is-it-a-make-a-transistor-opamp-like-lm358-chances-of-working/msg1403895/#msg1403895
 

Offline DmeadsTopic starter

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Amazing! thanks a lot! we just learned about the differential amplifier today!
 


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