# Differential pair, current mirror: high frequency analysis

• eliotsbowe
In summary, the post is discussing doubts about the high frequency response of MOSFET and BJT differential couples when polarized by a current mirror. The picture provided shows a semi-differential circuit and mentions that Cgdm is to be considered a bypass capacitor. The author also found a similar statement about BJT differential couples. The question is why there is a difference in the frequency response between the internal capacitances of the mirror and the main transistors. The answer is that the interaction between 2r0 and Cgdm creates a low frequency zero, making the other internal capacitances act as open circuits.
eliotsbowe
Hello, I have a doubt about the high frequency response of the MOSFET (or BJT as well) differential couple in the case it's polarized by a current mirror.

Here's a picture of my issue (it shows the semi-differential circuit of a MOSFET differential couple with a "small" common mode ac input; the current mirror was replaced by its high frequency small signal model)
http://s3.postimage.org/x84o1dsxv/asdasd.jpg

The picture says (sorry for posting a non-english text) that Cgdm is to be considered a bypass capacitor and the MOSFET should be replaced by its low frequency (!) small signal model.
The sense of this is that Cgdm "exists" at much lower frequencies than the internal capacitances of the MOSFETs composing the differential couple.

I found the same statement about the BJT differential couple (with a BJT-based current mirror connected to the emitter): the internal capacitance of the mirror dominates the low frequency response of the whole circuit and it acts like a bypass capacitor.

So, my question is: why is there such a difference between the "life" (on the frequency axis) of the capacitance of the mirror and the one of the internal capacitances of the two main transistors?

I found it. The Sedra-Smith's book says that the interaction between 2r0 and Cgdm creates a low frequency zero. Every other internal capacitance is to be considered as an open circuit, then.

## What is a differential pair?

A differential pair is a circuit configuration consisting of two transistors with their emitters connected together and their collectors connected to a common load. It is commonly used in electronic circuits to amplify differential signals and reject common mode signals.

## What is a current mirror?

A current mirror is a circuit that replicates the current of one transistor in another transistor. It is commonly used in electronic circuits to provide stable biasing currents and improve the accuracy of current sources.

## How is high frequency analysis performed on a differential pair?

High frequency analysis on a differential pair involves using small-signal models of the transistors and analyzing the circuit's response to small AC signals. This can help determine the circuit's bandwidth, gain, and other parameters at high frequencies.

## Why is it important to perform high frequency analysis on a differential pair?

High frequency analysis is important for differential pairs because it allows for accurate prediction of the circuit's performance at high frequencies. This can help ensure stability and proper functioning of the circuit in high frequency applications.

## What are some common applications of differential pairs and current mirrors?

Differential pairs and current mirrors are commonly used in electronic circuits such as operational amplifiers, analog-to-digital converters, and voltage regulators. They are also used in radio frequency circuits, audio amplifiers, and other high frequency applications.

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