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Cascode BJT Circuit

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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:- B e2 c2 C Vout Q2 c1 Zout b2 b1 RL Q1 Vin A e1 Zin Gain of Q1 due to load of Q2 1 CE gain = - gm1RL1 RIN (CB stage ) = gm2 1 ∴ A V 1 = - gm1 which assuming transistors are matched = 1 gm2 AI1 = - β Miller capacitance CSHUNT = CBC ()1− Av = CBC (1− (−1)) CSHUNT = 2CBC 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 β ICQ k.T -23 −1 RIN = where gm = ; VT = where k = Boltzmans constant = 1.3807x10 JK gm VT q q = Electron charge = 1.6022x10-19C T = Temperature in Kelvin gm = Transconductance (mS) 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: VOUT β2.ib2.RL β2.RL RL ICQ VA VA 1 A V = = = ≈ = . = (as = rce ) VIN ib2 (β2 + 1)rbe2 (β2 + 1)rbe2 rbe VT ICQ VT gm 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). VCC - VC 5 - 2.5 RL = = -3 = 500Ω ICQ 5x10 Device Data VA = 50V ; VT = 25mV ; β = 100 -3 VT 25x10 AV= - gm.R L = - .RL = - -3 .500 = 2500 in dB' s = 10 * log(2500) = 34dB ICQ 5x10 Vc - Vbe 2.5 - 0.7 Base collector bias resistor = = -3 = 36KΩ ICQ /β 5x10 /100 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. R V_DC R3 SRC1 DC R=500 Ohm Vdc=5.0 V DC DC1 I_Probe R I_Probe1 R4 R=27000 Ohm C AC B AC DC_Block E AC1 V_AC DC_Block1 Start=1.0 MHz SRC4 Stop=9000 MHz Vac=1 V HFA3127B_npn Step= Freq=freq X2 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 DC.vc2 DC.I_Probe1.i 2.289 V 5.423mA m2 m1 freq=1.000MHz freq=1.696GHz dB(AC.vc2)=35.944 dB(AC.vc2)=15.335 40m2 30 ) 20 m1 vc2 . C A ( 10 B d 0 -10 0123456789 freq, GHz 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. AC DC AC DC AC1 DC1 Start=1.0 MHz Stop=4000 MHz Step= V_DC R BJT_Model SRC1 R3 BJTM2 Vdc=5 V R=500 Ohm NPN=yes Br=1e1 Cjc=3.98e-13 Re=1.848 PNP=no Ikr=5.4e-2 Vjc=9.7e-1 Rc=1.14e1 R I_Probe Bf=1.036e2 Isc=1.6e-14 Mjc=2.4e-1 Kf=0 R5 I_Probe1 Ikf=5.4e-2 Nc=1.8 Xcjc=9e-1 Af=1 R=50 Ohm Ise=1.686e-19 Var=4.5 Fc=5e-1 Kb= Ne=1.4 Nr= Cje=2.4e-13 Ab= V_AC Vaf=7.2e1 Tr=4e-9 Vje=8.69e-1 Fb= SRC4 Nf= Eg=1.11 Mje=5.1e-1 Ffe= Vdc=0.817 V Tf=10.51e-12 Is=1.84e-16 Cjs=1.15e-13 Lateral=no Vac=1 V Xtf=2.3 Imax= Vjs=7.5e-1 AllParams= Freq=freq Vtf=3.5 Xti=3 Mjs=0 BJT_NPN Itf=3.5e-2 Tnom= Rb=5.007e1 BJT2 Ptf=0 Nk= Irb= Model=BJTM2 Xtb=0 Iss= Rbm=1.974 Area= Approxqb=yes Ns= RbModel=MDS Region= Temp= Mode=nonlinear Sheet 5 of 6 The resulting plot now shows how gain has rolled off faster due to the Miller effect. DC.vc2 DC.I_Probe1.i 2.486 V 5.028mA m2 m1 freq=1.000MHz freq=1.692GHz dB(AC.vc2)=34.820 dB(AC.vc2)=8.738 40m2 30 ) vc2 . 20 C A ( B m1 d 10 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 freq, GHz 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. AC DC AC AC1 DC Start=1.0 MHz DC1 Stop=4000 MHz Step= V_DC R SRC1 R3 Vdc=5 V R=500 Ohm I_Probe BJT_Model I_Probe1 BJTM2 NPN=yes Br=1e1 Cjc=3.98e-13 Re=1.848 BJT_NPN PNP=no Ikr=5.4e-2 Vjc=9.7e-1 Rc=1.14e1 BJT3 V_DC Bf=1.036e2 Isc=1.6e-14 Mjc=2.4e-1 Kf=0 Model=BJTM2 SRC5 Ikf=5.4e-2 Nc=1.8 Xcjc=9e-1 Af=1 Area= Vdc=3 V Ise=1.686e-19 Var=4.5 Fc=5e-1 Kb= Region= Ne=1.4 Nr= Cje=2.4e-13 Ab= R Temp= Vaf=7.2e1 Tr=4e-9 Vje=8.69e-1 Fb= R5 Mode=nonlinear Nf= Eg=1.11 Mje=5.1e-1 Ffe= R=50 Ohm Tf=10.51e-12 Is=1.84e-16 Cjs=1.15e-13 Lateral=no Xtf=2.3 Imax= Vjs=7.5e-1 AllParams= V_AC Vtf=3.5 Xti=3 Mjs=0 SRC4 Itf=3.5e-2 Tnom= Rb=5.007e1 Vdc=0.817 V Ptf=0 Nk= Irb= Vac=1 V Xtb=0 Iss= Rbm=1.974 Freq=freq Approxqb=yes Ns= RbModel=MDS BJT_NPN BJT2 Model=BJTM2 Area= Region= Temp= Mode=nonlinear 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.vc2 DC.I_Probe1.i 2.528 V 4.944mA m1 m2 freq=1.000MHz freq=1.692GHz dB(AC.vc2)=34.935 dB(AC.vc2)=12.631 40m1 30 ) ) 2 c vc1 v . C C 20 A A ( ( m2 B B d d 10 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 freq, GHz .
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