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AE_Lecture6_Analysis of CASCODE AMPLIFIER.

Module by: Bijay_Kumar Sharma. E-mail the author

Summary: AE_lecture6 deals with midfrequency and high frequency analysis of CASCODE Amplifier.

AE_LECTURE 6_Analysis of CASCODE AMPLIFIER

CASCODE AMPLIFIER- This is a form of composite transistor where CE and CB have been cascaded. By using the composite form we achieve best of both the circuit configurations namely we get a moderate input impedance and high voltage gain of CE configuration and almost near unilaterality , very large output impedance , large output voltage swing limited by BVCBO and much larger Band Width of CB configuration. Because of near uni-laterality on account of near zero reverse transmission factor, this CASCODE is very suitable RF Amplifier applications. This configuration is also suitable for constant current drive as required in generating SAW TOOTH WAVEFORM.

Figure 1
Figure 1 (Picture 1.png)

This CASCODE configuration has a self biasing for Q point stabilization. Both Transistors are in forward active mode. Q1 is connected in CE configuration under signal condition and Q2 is in CB configuration under signal condition.

C1 provides the ground to Q2 under signal condition.

C2 is the coupling capacitor and C3 provides the by-pass capacitor of emitter resistance.

INCREMENTAL MODEL

Figure 2
Figure 2 (Picture 2.png)

Effectively we obtain CE configuration followed by CB configuration hence we call it CE-CB cascade.

Overall Reverse transmission factor= hre.hrb

=

Figure 3
Figure 3 (graphics1.png)
= 10-4×10-5 = 10-9= this provides the near-unilaterality property to CASCODE configuration making it suitable for RF applications.

Overall Rout=1/hob=2M

Overall Rin=hie=rx+rπ=(100+2.6kΩ)

Frequency response of CB>> Frequency response of CE

CE configuration faces a load which is Rin of CB which is re . Hence Miller Muliplication factor is only 1+1 hence Miller Capacitance is much lower thereby boosting the frequency response of CASCODE configuration.

INCREMENTAL MODEL

Figure 4
Figure 4 (Picture 5.png)

Rs=1kΩ, βfo=100, rx=0.1k, Cµ=2pF, RL=2k, gm=40mS, rπ=2.5k, Cπ=100pF

Figure 5
Figure 5 (graphics2.png)
Figure 6
Figure 6 (graphics3.png)
Figure 7
Figure 7 (graphics4.png)
Figure 8
Figure 8 (graphics5.png)
Figure 9
Figure 9 (graphics6.png)
Figure 10
Figure 10 (graphics7.png)
Figure 11
Figure 11 (graphics8.png)

AT MID FREQUENCIES

Figure 12
Figure 12 (Picture 6.png)

Figure 13
Figure 13 (graphics9.png)
Figure 14
Figure 14 (graphics10.png)
Figure 15
Figure 15 (graphics11.png)
Figure 16
Figure 16 (graphics12.png)
Figure 17
Figure 17 (graphics13.png)
Figure 18
Figure 18 (graphics14.png)
Figure 19
Figure 19 (graphics15.png)
Figure 20
Figure 20 (graphics16.png)
Figure 21
Figure 21 (graphics17.png)
Figure 22
Figure 22 (graphics18.png)

Midband gain of cascade ~ a single stage CE amplifier with a load resistance of 2k.

BW calculations

The midband gain of CE stage =-gmre=-1

Therefore Miller Capacitance=Cµ[1-(-1)]=2Cµ

Therefore total input Capacitance=Cπ+Cµ (1-AV)=100+8=108pF

There is very little Miller Multiplication of Cµ.

Req of CM (Miller Capacitance) is =

Figure 23
Figure 23 (graphics19.png)

Figure 24
Figure 24 (graphics20.png)
Figure 25
Figure 25 (graphics21.png)
Figure 26
Figure 26 (graphics22.png)

The BW of common base is much larger.

Figure 27
Figure 27 (graphics23.png)
Figure 28
Figure 28 (graphics24.png)
Figure 29
Figure 29 (graphics25.png)
Figure 30
Figure 30 (graphics26.png)
Figure 31
Figure 31 (graphics27.png)
Figure 32
Figure 32 (graphics28.png)
Figure 33
Figure 33 (graphics29.png)

Overall BW of Cascode=1.93Mhz

Exact analysis gives the same result.

Figure 34
Figure 34 (graphics30.png)

Four poles because there are Cµ,Ce,Cπ,Cbo.

There are two zeroes which are complex conjugate.

If CB stage was replaced by 2k to obtain the same gain the Miller Multiplication would have increased and BW fallen to 491 kHz.

Figure 35
Figure 35 (graphics31.png)
Figure 36
Figure 36 (graphics32.png)

fh=491kHz

(1)Large gain x BW or large GBP

(2)Output Voltage swing is limited by BVCBO and not BV CEO.

Applications:-

(1)Wide band video amplifier(TV & FM Radio)

(2)Wide Band Amplifier used in RF communication,

(3)Near ideal current sources and in high gain amplifiers.

(4)Current Amplification factor is increased(βfo +1).

(5)Used in high performance differential amplifers which is the building block of op amps. This enables very high gain & high CMRR.

(6)We can realize near ideal current sources.

(7)Much higher B.W.

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