and a simplified large signal model of a transistor,
respectively.
(i) Assuming the transistor is in the active region, redraw the
circuit in Figure 2(a) by using the simplified large signal model
(Figure 2b) in place of the transistor, and derive an expression
relating VOUT to VIN. [4 marks]
(ii) Based on the expression in a(i), sketch the voltage transfer
characteristics curve, i.e., VOUT vs VIN. Comment on the
limitations of the simplified large signal model. [3 marks] (iii)
Assuming VBE(on) = 0.7V and VCE(saturation) = 0.2V, redraw your
sketch to include its cut-off, active and saturation regions. Mark
on this sketch a suitable quiescent operating point for the circuit
and explain your reasoning. [3 marks]
(b) The transistor amplifier circuit is now biased with a
voltage divider circuit, R1 and R2, coupling capacitors, C1 and C2
and an emitter resistor RE as shown in Figure 3 a.
(i) What is the role of resistor RE? [2 marks]
(ii) Using a Thevenin equivalent circuit to simplify the biasing
circuit, find IC and show that the circuit is now biased such that
the quiescent point is approximately in the centre of the load
line. [6 marks](iii) Given that gm = 40IC and rπ = β/gm, use the
value of IC from b(ii) to find rπ. Using a simplified hybrid-Pi
model determine the AC input impedance of the amplifier circuit. [3
marks]
(iv) Describe a situation where the input impedance from b(iii) may
cause the circuit to underperform. Suggest a way to improve the
situation. [4 marks]
- A2 A Figures 2 A And 2 B Show A Simple Transistor Amplifier And A Simplified Large Signal Model Of A Transistor Res 1 (37.32 KiB) Viewed 55 times