Wide-band Differential Amplifier Calculator

The wide-band differential amplifier has high gain and high band width. It overcomes the Miller capacitance limitations of the common emitter amplifier by tying the collector of Q1, and the base of Q2 to signal ground. As a result it can achieve gain bandwidths orders of magnitude larger than the common emitter amplifiers. The only cost is that a second transistor is needed.

This calculator computes bias voltage and current levels, as well as gain and frequency response.

R1	(K ohms)
R2	(K ohms)
RC (Collector resistor)	(K ohms)
RE (Emitter resistor)	(K ohms)
VP (Supply Voltage)	(V)
Beta (DC Current Gain)
VBE (Base to emitter drop)
Rs (Source Resistance):	(ohms)
RL (Load resistor)	(ohms)
fT (Current Gain BW Product):	(MHz)
CCB (Cu Collector-Base Cap.):	(pF)
CBE (Cπ, Base-Emitter Cap.): Optional	(pF)
VC (Collector Voltage)	(V)
VE(Emitter Voltage)	(V)
VB (Base Voltage)	(V)
IE (Emitter Voltage)	(mA)
IC (Collector Voltage)	(mA)
IB (Base Voltage)	(mA)
gm (Transconductance)
rπ (Input Resistance of BJT at low freq)	(ohms)
re	(ohms)
RIN(Input Resistance of Amp)	(K ohms)
A (Amplifier Voltage Gain)
fP1	(MHz)
fP2	(MHz)

Equations

V_B= VP*R2/(R1+R2)

V_E= V_B-V_BE

I_E=V_E/R_E

Alpha= Beta/(Beta+1);

I_C= Alpha*I_E

V_C= VP - I_C*R_C/2

I_B=I_C/Beta

g_m = Ic/25mA

r_e= Alpha/g_m

r_π=Beta/g_m

A=Alpha* r_π*R_C/(R_s+2*r_π)/r_e

f_P1=1/(2*π*(R_s||2*r_π)*(C_BE/2+C_BC))

f_P2=1/(2*π*R_C*C_BC)

f_T=g_m/(2π*(C_BE+C_BC))

The lower pole is dominate.