In a cascode current source why the current buffer transistor is immune to temparature fluctations

ian field

Joined Oct 27, 2012
6,536
It doesn't. Your drawing says, "improved stability with load voltage variation", not, "perfect stability with temperature variation".
It wasn't clear to me why adding a common base stage was particularly an improvement - it does increase the output impedance, but the current gain is slightly less than unity.

The first improvement I'd make, is a stabilised voltage reference for the base of the bottom transistor - if temperature stability is important, its possible to incorporate compensation into the voltage reference.
 

#12

Joined Nov 30, 2010
18,224
The first improvement I'd make, is a stabilized voltage reference for the base of the bottom transistor - if temperature stability is important, its possible to incorporate compensation into the voltage reference.
I hope you enjoy explaining that to the TS.
 

Thread Starter

Himanshoo

Joined Apr 3, 2015
265
It doesn't. Your drawing says, "improved stability with load voltage variation", not, "perfect stability with temperature variation".
It looks like i got the story(though i picked the concept late) … Suppose load voltage varies then Vce_Q2 varies in opposite direction in order to compensate changes in load voltage just like a single transistor acting as a current source does.Q2 by doing so stabilises the collector current which means a stable voltage at the collector terminal of Q1 and further a stable Vce for Q1.....Q1..kind of compensating early effect—->which involves a change in Ic with a change in Vcb or (Vce)..Q2 acting as a buffer has just a role of isolating Q1 from load and load voltage variations…which indeed don't depend on temperature....:D

probably now i am right ??
 

crutschow

Joined Mar 14, 2008
34,285
Below is the simulation of the circuit with a temperature sweep from 25°C to 100°C.
The change of current with this 75°C temperature change is over 10% so obviously the circuit is not temperature compensated.
So I'll let you decide if you are right. :rolleyes:

Temp Sweep.gif
 
Last edited:

OBW0549

Joined Mar 2, 2015
3,566
It looks like i got the story(though i picked the concept late) … Suppose load voltage varies then Vce_Q2 varies in opposite direction in order to compensate changes in load voltage just like a single transistor acting as a current source does.Q2 by doing so stabilises the collector current which means a stable voltage at the collector terminal of Q1 and further a stable Vce for Q1.....Q1..kind of compensating early effect—->which involves a change in Ic with a change in Vcb or (Vce)..Q2 acting as a buffer has just a role of isolating Q1 from load and load voltage variations…which indeed don't depend on temperature....:D

probably now i am right ??
The cascode circuit configuration has absolutely nothing whatsoever to do with temperature stability, and never has.

The cascode configuration, as an enhancement to the ordinary common-emitter amplifier, does two things, either or both of which may be important in any given application:

First, by drastically reducing the collector voltage variation in the common-emitter stage (the lower transistor, the one that provides all the voltage gain), the common-base stage (the upper transistor) reduces the Early effect to insignificance. This has three consequences: 1) the output impedance is increased, 2) maximum available voltage gain is increased, and 3) distortion is somewhat reduced (this latter effect may or may not be significant, depending on the design).

And second, for the same reason, Miller effect capacitance is reduced, dramatically increasing the amplifier bandwidth.

But one thing the cascode configuration absolutely does NOT do, is have much effect on the temperature stability of the circuit, one way or the other.
 
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