Swamping Ce Amplifier

Swamping of a Common Emitter Amplifier The term “swamping” refers to the practice of having the emitter capacitor CE , referred to as a bypass capacitor, connected in parallel with only a portion of the total resistance in the emitter lead of the transistor amplifier. The diagram below shows a Common Emitter amplifier with a swamping capacitor. The 100 Ω is the swamping portion of the total emitter resistance of 1100 Ω.

The DC bias voltages and currents are: VB = 2.12 V

VE = 1.42 V

VC =

VCE = 5.74 V – 1.42 V = 4.32 V

10 V – 4.26 V = 5.74 v

IE = IC = 1.29 mA

The AC dynamic emitter resistance re’ = 25 mV/IE = 19.4 Ω Recall that the value of re’ (like β ) varies from transistor to transistor and is also sensitive to changes in temperature. Let’s assume for purposes of this handout that re’ varies from 15 Ω to 25 Ω. The table below summarizes the effect of a changing re’ on the voltage gain of the amplifier under different swamping conditions. The voltage gain AV = - RC||RL / (re’ + RE) where RE is that part of the emitter resistance that is not in parallel with CE. It is the swamped value of RE.

re’ value

15 Ω 19.4 Ω 25 Ω

AV - no CE - fully swamped RE value = 1100 Ω 2.22 2.22 2.20 % variation ≈ 1 %

AV - CE present partially swamped RE value = 100 Ω 21.6 20.8 19.8 % variation = 9.1 %

AV - CE present - no swamping RE value = 0 Ω 165 128 99.2 % variation = 66 %

DC and AC grounds The DC ground point is at the bottom of the emitter resistor. With the CE capacitor connected the circuit now has an AC ground point at the top of the CE capacitor – shorts AC signal to “ground”. Swamping the AC dynamic emitter resistance re’ has two effects •

Reduces the voltage gain

•

Reduces the variability of voltage gain from one circuit to another – stabilizes the voltage gain