# Designing a common base amplifier to meet given specifications

I designed the circuit as below and I know that common base amplifier should have a unity current gain(usually <1) but I'm getting a very high gain. What might be the reason for this, is something wrong with the topology?

DC op simulation:

• Manan, if you are done with this question you should select an answer for acceptance by pressing the answer accept button. Maybe review your earlier questions too in case this message might also apply to them. Commented Apr 2, 2021 at 17:14

but I'm getting a very high gain

I don't see what you see: -

Realize that a Common Base is a current amplifier (with a gain of <1 so not really an amplifier) OK, current buffer.

But still: current in => current out.

You're using a voltage source Vin and a resistor (voltage => current), that's OK but realize that this could influence that transfer function of the common base unless you really probe the current into the emitter.

Pro tip: what I would do is remove Vin and R1 and use a current source. Actually I would use two current sources in parallel. One for the DC bias current and a 2nd current source for the signal current. Yes, you can also combine the same in one source if you prefer (AC = 1, DC = 50 uA).

At the output it is the same, a current comes out but where can it go? You have there the output of a current mirror which ideally provides a constant current. That constant current will be "fighting" against the non-constant current from the collector.

In general you would need to terminate a current output with a low impedance point like a voltage source to ground.

Here's how I would simulate the behavior of a common base circuit:

simulate this circuit – Schematic created using CircuitLab

The input us now a current: Iin

The output is also a current, the current through Vcurrentsense

If you want to change the Collector-Emitter DC voltage $$\V_{CE}\$$ then simply adjust the DC value of Vcurrentsense.

Note how there are no resistors in my schematic! So there are no resistors to influence the transfer function. If you do want to see the effect of certain resistors then you can add them of course.