Design common base amplifier, beta independent

A common base (CB) amplifier with a voltage divider (β-independent) bias holds the base at fixed dc voltage and grounds it for ac with a capacitor CB. The input is at the emitter and the output at the collector.

Guidelines

1. VE ≈ VCC/10.
2. VCE ≈ VCC/2 (symmetrical swing).
3. Divider current I2 = 5 to 10 IB; for a comparatively large Rin and Rout, take a large RE and RC and a divider current at the lower end: I2 = 5 IB.
4. VB = VE + 0.7 V; CB bypasses the base to ground at signal frequency.

Design (VCC = 22 V, IE = 1.5 mA, β = 110)

IC = α IE = (110/111)(1.5 mA) = 1.486 mA        IB = IE/(β+1) = 13.5 µA
VE = VCC/10 = 2.2 V          RE = VE/IE = 2.2/1.5 mA = 1.47 kΩ
VCE = VCC/2 = 11 V           RC = (22 - 2.2 - 11)/1.486 mA = 5.92 kΩ
VB = 2.2 + 0.7 = 2.9 V
I2 = 5 IB = 67.6 µA
R2 = VB/I2 = 2.9/67.6 µA = 42.9 kΩ
R1 = (22 - 2.9)/(67.6 + 13.5) µA = 235.6 kΩ

Choose R1 ≈ 240 kΩ, R2 ≈ 43 kΩ, RE ≈ 1.5 kΩ, RC ≈ 5.9 kΩ (about 6 kΩ). CB is a large bypass capacitor from base to ground; C1, C2 are the coupling capacitors.

Check (small signal)

re  = VT/IE = 25 mV/1.5 mA = 16.7 Ω
Rin = RE || re = 16.5 Ω   (low: the property of CB)
Rout ≈ RC = 5.92 kΩ        (high)
Av  = RC/re = 5.92 kΩ/16.7 Ω ≈ 355  (no load)

Result: R1 = 240 kΩ, R2 = 43 kΩ, RE = 1.5 kΩ, RC = 5.9 kΩ; Av ≈ +355 (non-inverting); Rout ≈ 5.9 kΩ. The input resistance of a CB stage is inherently small (about re); RE is chosen large so that it does not lower it further.