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

Discussion in 'General Electronics Chat' started by Himanshoo, May 27, 2015.

1. ### Himanshoo Thread Starter Member

Apr 3, 2015
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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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2. ### studiot AAC Fanatic!

Nov 9, 2007
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See Here

http://www.allaboutcircuits.com/vol_3/chpt_4/13.html

The cascode configuration is a common emitter followed by a common base (roughly) the input impedance is that of the common emitter input (medium) and the output impedance is that of the common base output (high).

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3. ### Himanshoo Thread Starter Member

Apr 3, 2015
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which transistor should i consider in CB configuration??is it Q1

4. ### ian field AAC Fanatic!

Oct 27, 2012
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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.

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5. ### Himanshoo Thread Starter Member

Apr 3, 2015
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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)

6. ### crutschow Expert

Mar 14, 2008
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The Wilson mirror is a three transistor current mirror with significantly improved characteristics also, as compared to the two transistor mirror.

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7. ### t_n_k AAC Fanatic!

Mar 6, 2009
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@Himanshoo : I would suggest Q4 rather than Q1.

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8. ### Himanshoo Thread Starter Member

Apr 3, 2015
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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..

9. ### Bordodynov Well-Known Member

May 20, 2015
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I compared the 5 current generators. The picture shows the output currents and the output dynamic resistance.

• ###### current_source.JPG
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10. ### t_n_k AAC Fanatic!

Mar 6, 2009
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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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