cascode bjt circuit

Sheet 1 of 6

Cascode BJT Circuit A popular circuit for use at high frequencies is the cascode amplifier, as shown below:-

b1

e1

c1

Vin

Vout

Zin

Zout

RL

b2

e2 c2 Q2

Q1

B C

A

Gain of Q1 due to load of Q2

β- A

1 matched are stransistor assuming whichgm

1gm- A

gm1 ) stage (CB R Rgm- gain CE

I1

211V

2INL11

=

==∴

==

Miller capacitance

( ) ( ) BCSHUNTBCvBCSHUNT 2C C )1(1C A1C C =−−=−= The miller capacitance across the input of the CE stage is double the Base collector capacitance of Q1.

Sheet 2 of 6

Input Impedance of Q1 at point A is

(mS) ctanceTranscondu Kelvin in eTemperatur T

1.6022x10 charge Electron q

1.3807x10 constant Boltzmans k whereq

k.T V; VI whereβR

19-

123-T

T

CQIN

==

==

===== −

gm

C

JKgmgm

Voltage Gain Av Voltage gain of CE stage is ~ 1 (due to low output RL) so the voltage gain of the amplifier will be from the common-base stage:

)rgm1 (as

VV

IV.

VI

rR

1)r(βR.β

1)r(βiR.i.β

VVA ce

T

A

CQ

A

T

CQ

be

L

be22

L2

be22b2

Lb22

IN

OUTV ===≈

+=

+==

Current Gain Ai Current gain of CB stage is ~ 1 so the current gain of the amplifier will be from the common-emitter stage. Current gain (Ai) = Ai of the CE stage (Ai of CB stage = 1) = β Output Impedance ROUT = RL Of course as the voltage gain of the whole amplifier is dependant on the load resistor (and ultimately rce2), then adding an active load (eg current mirror) will allow high voltage gain.

Sheet 3 of 6

Cascode Simulation For comparison a common-emitter stage was simulated on ADS to measure voltage gain. Using the HF3127B transistor array we can calculate the voltage gain of the circuit at low frequencies. If we assume a supply voltage of 5V and a device current of 5mA, we can calculate the value of the load resistor (We also assume that we want half the supply voltage across the device so that VC = 2.5V).

Ω===

======

===

Ω===

36K /1005x100.7 - 2.5

/βI Vbe-Vc resistor bias collector Base

34dB log(2500)*10 sdB' in 2500 500.5x1025x10- R.

IV

- gm.R- AV

100 β ;25mV V;50V VData Device

500 5x10

2.5 - 5 I

V- V R

3-CQ

3-

3-

LCQ

TL

TA

3-CQ

CCCL

At low frequencies we would expect a voltage gain of 34dB rolling off to a gain of 0dB at the fT of the device (~ 8GHz). Below is shown the simulation and result from ADS.

HFA3127B_npnX2

B

C

E

V_ACSRC4

Freq=freqVac=1 V

DC_BlockDC_Block1

ACAC1

Step=Stop=9000 MHzStart=1.0 MHz

AC

RR4R=27000 Ohm

RR3R=500 Ohm

DCDC1

DC

I_ProbeI_Probe1

V_DCSRC1Vdc=5.0 V

Note a DC block is used as the AC source has a DC of 0V. During simulation the value of the base-collector resistor was reduced to ensure IC = 5mA. And the resulting data of voltage gain vs frequency for the single-stage common-emitter amplifier.

Sheet 4 of 6

m1freq=1.696GHzdB(AC.vc2)=15.335

m2freq=1.000MHzdB(AC.vc2)=35.944

0 1 2 3 4 5 6 7 8 9-10

0

10

20

30

40

freq, GHz

dB(A

C.v

c2)

m1

m2

DC.vc22.289 V

DC.I_Probe1.i5.423mA

The predicted low-frequency voltage gain agrees well with our initial calculation. However, we have not included a source resistance – this will effectively form a potential divider with the base bulk resistor rb, and lower the voltage entering the device and hence lowering the gain. Now a 50-ohm resistor has been added to the ideal input voltage source.

BJT_ModelBJTM2

AllParams=Lateral=noFfe=Fb=Ab=Kb=Af=1Kf=0Rc=1.14e1Re=1.848

RbModel=MDSRbm=1.974Irb=Rb=5.007e1Mjs=0Vjs=7.5e-1Cjs=1.15e-13Mje=5.1e-1Vje=8.69e-1Cje=2.4e-13Fc=5e-1Xcjc=9e-1Mjc=2.4e-1Vjc=9.7e-1Cjc=3.98e-13

Ns=Iss=Nk=Tnom=Xti=3Imax=Is=1.84e-16Eg=1.11Tr=4e-9Nr=Var=4.5Nc=1.8Isc=1.6e-14Ikr=5.4e-2Br=1e1

Approxqb=yesXtb=0Ptf=0Itf=3.5e-2Vtf=3.5Xtf=2.3Tf=10.51e-12Nf=Vaf=7.2e1Ne=1.4Ise=1.686e-19Ikf=5.4e-2Bf=1.036e2PNP=noNPN=yes

V_ACSRC4

Freq=freqVac=1 VVdc=0.817 V

DCDC1

DCACAC1


AC

BJT_NPNBJT2

Mode=nonlinearTemp=Region=Area=Model=BJTM2

V_DCSRC1Vdc=5 V

RR3R=500 Ohm

I_ProbeI_Probe1

RR5R=50 Ohm

Sheet 5 of 6

The resulting plot now shows how gain has rolled off faster due to the Miller effect.



0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00

10

20

30

40

freq, GHz

dB(A

C.v

c2)

m1

m2

DC.vc22.486 V


To improve the high frequency response and stability we now add a common-base stage to form a Cascode amplifier. The common-base stage is biased in saturation and therefore there will be little voltage drop across the collector-emitter junction.

ACAC1


AC

DCDC1

DC

BJT_ModelBJTM2

AllParams=Lateral=noFfe=Fb=Ab=Kb=Af=1Kf=0Rc=1.14e1Re=1.848

RbModel=MDSRbm=1.974Irb=Rb=5.007e1Mjs=0Vjs=7.5e-1Cjs=1.15e-13Mje=5.1e-1Vje=8.69e-1Cje=2.4e-13Fc=5e-1Xcjc=9e-1Mjc=2.4e-1Vjc=9.7e-1Cjc=3.98e-13

Ns=Iss=Nk=Tnom=Xti=3Imax=Is=1.84e-16Eg=1.11Tr=4e-9Nr=Var=4.5Nc=1.8Isc=1.6e-14Ikr=5.4e-2Br=1e1

Approxqb=yesXtb=0Ptf=0Itf=3.5e-2Vtf=3.5Xtf=2.3Tf=10.51e-12Nf=Vaf=7.2e1Ne=1.4Ise=1.686e-19Ikf=5.4e-2Bf=1.036e2PNP=noNPN=yes

V_DCSRC1Vdc=5 V

RR3R=500 Ohm

I_ProbeI_Probe1

BJT_NPNBJT3


V_DCSRC5Vdc=3 V

V_ACSRC4

Freq=freqVac=1 VVdc=0.817 V

BJT_NPNBJT2


RR5R=50 Ohm

Sheet 6 of 6

The red plot shows the voltage gain of the CE stage showing that the gain of the first stage is now a lot lower than the single CE stage amplifier. The blue plot shows the new voltage gain response of the Cascode amplifier and at marker 2 (1.69GHz) the gain has improved from 8.7dB to 12.6dB, as the Milller effect has been reduced.

DC.vc22.528 V




0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00

10

20

30

40

freq, GHz

m1

m2

dB(A

C.v

c2)

dB(A

C.v

c1)

cascode bjt circuit

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