Transistor as an Amplifier
The Transistor as an Amplifier
– Amplification is the process of linearly increasing the amplitude of an electrical signal.
– A transistor can act as an amplifier directly using the gain, b.
– Keep in mind that when a transistor is biased in the active (linear) region, the BE junction has a low resistance due to forward bias and the BC junction has a high resistance due to reverse bias.
i) DC and AC quantities
– Amplifier circuits have both ac and dc quantities.
– Capital letters are used will be used for both ac and dc currents.
– Subscript will be capital for dc quantities.
– Subscript will be lowercase for ac quantities.
ii) Transistor amplification
– A transistor amplifies current because the collector current is equal to the base current multiplied by the current gain, b.
– Base current (IB) is small compared to IC and IE.
– Thus, IC is almost equal to IE.
– Consider the following circuit.
– An ac voltage, Vin, is superimposed on the dc bias voltage VBB.
– DC bias voltage VCC is connected to the collector through the collector resistance, RC.
– The ac input voltage produces an ac base current, which results in a much larger ac collector current.
– The ac collector current produces an ac voltage across RC, thus producing an amplified, but inverted, reproduction of the ac input voltage in the active region.
– The forward biased base-emitter junction present low resistance to the ac wave.
– This internal ac emitter resistance is designated r’e.
Ie ? Ic = Vb/ r’e
– The ac collector voltage, Vc = IcRC.
– Since Ie ? Ic, the ac collector voltage is Vc ? IeRC.
– Vb can be considered the transistor ac input voltage where Vb = Vin – IbRB.
– Vc can be considered the transistor ac output voltage.
– The ratio of Vc to Vb is the ac voltage gain, Av, of the transistor circuit.
Av = Vc/Vb
– Substituting IeRC for Vc and Ie r’e for Vb yields
Av = Vc/Vb ? (IeRC)/(Ie r’e) = RC/ r’e
– Thus, amplification depends on the ratio of RC and r’e.
– RC is always considerably larger in value than r’e, thus the output voltage is larger than the input voltage.
Determine the voltage gain and the ac output voltage for the following circuit if r’e = 50 W.
The voltage gain is
Av ? RC/r’e = 1 k W /50 W = 20
Thus the output voltage is
Vout = AvVb = (20)(100 mV) = 2 Vrms