Advantages of cascoding ........how??

Thread Starter

Himanshoo

Joined Apr 3, 2015
259
In order to improve the output impedance of a current mirror emitter resistance are added to them..but on the other way these emitter resistances have certain drawbacks which involve restricting output voltage to a lower range etc..and then in order to nullify these disturbances cascode stages of transistors are required..which provides a much higher output impedances without large emitter voltage drops...

But how does cascode configuration improves output impedance to a higher level ???.........can't exactly imagine??
 

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ian field

Joined Oct 27, 2012
6,543
which transistor should i consider in CB configuration??is it Q1
View attachment 86417
The common base stage usually has a fixed reference voltage applied to the base.

Cascode amplifiers were usual in PC monitor video amplifiers, the high output impedance wasn't that good for driving the capacitance of CRT cathodes, so all the good ones were immediately followed by a comlementary pair of emitter followers.

Many CRT sub-panels had something like a 7808 regulator that served no other purpose than providing the 3 common base parts of the cascodes with base voltage.
 

Thread Starter

Himanshoo

Joined Apr 3, 2015
259
The common base stage usually has a fixed reference voltage applied to the base.

Cascode amplifiers were usual in PC monitor video amplifiers, the high output impedance wasn't that good for driving the capacitance of CRT cathodes, so all the good ones were immediately followed by a comlementary pair of emitter followers.

Many CRT sub-panels had something like a 7808 regulator that served no other purpose than providing the 3 common base parts of the cascodes with base voltage.
so it must be Q1 because it is shorted ..having same potential at two ends of short wire..thus we can regard them to be at zero volts...(as a reference)
 

Thread Starter

Himanshoo

Joined Apr 3, 2015
259
@Himanshoo : I would suggest Q4 rather than Q1.
As Q2 is common emitter and Q4 you are considering as common base since it is fixed at a reference voltage( 0 V) that is solely decided by programming resistor R.
Am i right ..
but then what about Q3 ..since BASE of Q3 is also at same potential as of Q4 so..can we also assume Q3 to be in CB configuration..
 

t_n_k

Joined Mar 6, 2009
5,448
As Q2 is common emitter and Q4 you are considering as common base since it is fixed at a reference voltage( 0 V) that is solely decided by programming resistor R.
Am i right ..
but then what about Q3 ..since BASE of Q3 is also at same potential as of Q4 so..can we also assume Q3 to be in CB configuration..
For the purposes of small signal analysis Q4 is regarded as having a 'grounded' base by virtue of there being an almost constant DC base voltage. This small signal concept is really only "useful" for the evaluation of the source output resistance - or that resistance seen looking back into Q4 collector. If \[\text{r_{o}}\] is the Early Effect dependent output resistance for Q4 with current gain \[\text{\beta}\] then the source output resistance is approximated by \[\text{\beta r_{o}}\]. One can probably also use this small signal model to evaluate the small signal frequency response of the output current with respect to the reference current. An important caveat on the validity of this analysis is that one generally assumes all transistors are matched.
The fact that Q3's base voltage is the same as that of Q4's base probably has little or no impact on the source output resistance.
 
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