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ASSESSMENT ELEC202PA
Answer FOUR questions
MOSFET formulas
. . 2In = K[2(vGS - V;)vns -vns ]
. 2 k' (W)( lln = K (VGS - V;) = 2 L vGS - V;
K=!JlC (W) k' = JlCox2 ox L
gm = 2K(vGS .:..-V;)
IVAI
r =
o I
n
BJT formulas
. 1( VBE
le = IsexPl V )
T
·ie =aiE ie = !JiB iB= (l-a)iE
iE = (jJ+l)iB
!J=~ a=~
I-a !J+l
le . V VTr =.-.L r =gm=V: 1r I e I
T B E
V
r =~
o I
e
Page 1 of7
TURNOVER
--
ELEC202P
1.
Figure 1.1 shows a common-source amplifier circuit. CCl = Cs = CC2 = 1 ~F, RG = la MO,
Rsig = 100 kO, gm = 2 mAlV, Rn = RL = la kO, and Iv = O.
(a) At mid-band, CCl, Cs and CC2 can be treated as short circuit. Draw the small-signal
equivalent circuit ofthe CS amplifier at mid-band using the hybrid-n model.
[5 marks]
(b) Find the mid-band voltage gain, AM = Vo
Vsig
[5 marks]
(c) Determine the pole frequencies, COPl, COP2 and COP3 which are due to the presence of CCl, Cs
and CC2, respectively.
[9 marks]
(d) Which of the capacitors (CCl, Cs or CC2) contributes to the dominant pole?
[2 marks]
(e) Express the transfer function, A(s), of the common-source amplifier using AM, COPl, COP2 and
COP3.
[4 marks]
CC::'.
-------1l-----u I/o
CCI {(;i!!,
-----11__--Wv------o----II- c:s
r
\ / :-.ig ~-
I
- -
Figure 1.1
Page 2 of7
CONTINUED
ELEC202P
2.
The common-emitter amplifier shown in Fig. 2.1 has Rc = 8 kO. The transistor has gm =
40 mAIV, Yx = 50 0, Yn = 2.5 kO, Yo = 100 kO, Cn = 7 pF, and Cfl = 1 pF. The amplifier is fed
from a voltage source with an internal resistance (i.e., Rsig) of 5 kO. Assume the amplifier is
loaded by a purely resistive load at its output with RL = 5 kO.
(a) Draw the small-signal equivalent circuit of the CE amplifier using the high-frequency
hybrid-n model for the BJT.
[6 marks]
(b) Find theoverall mid-band voltage gain, AM. (You needto take into account the effect of
channel length modulation.)
[4 marks]
(c) Apply the Miller's theorem to Cfl and calculate the individual corner (or break or pole)
frequencies,fpi andfpo at the input and output sides, respectively.
[6 marks]
(d) Use the open-circuit time-constant method to find the individual corner (or break or pole)
frequencies,}; andh, which are due to the presence of Cn and Cfl' respectively.
[6 marks]·
o(e) Express the high-frequency transfer function, A(s) = v , of the CE amplifier using AM,fpi
Vsig
andfpo.
[3 marks]
-vss
Figure 2.1
Page 3 of7
TURNOVER
ELEC202P
3.
(a) Which amplifier configuration is adopted in Fig. 3.1? (Choose from CS, CO and CD.)
[3 marks]
Cb) Which feedback topology is employed in Fig. 3.1? (Choose from voltage amplifier, current
amplifier, transconductance amplifier and transresistance amplifier.)
[3 marks]
(c) What will be the other name for the feedback topology in Fig. 3.17 (Choose from shunt-shunt,"
shunt-series, series-series and series-shunt.) .
[3 marks]
(d) Explain why the feedback is negative.
[4 marks]
(e) Assume Ri + Rz» RD and neglect ro.
i) Find the expression for the open-loop gain A = Vo / Vi.
[3 marks]
ii) Find the expression for the feedback factor, fJ = Vf / Vo•
[3 marks]
iii) Find the expression for the closed loop gain, Af = Vo / Vs?
[3 marks]
iv) At what condition is Af approximately equal to lIfJ?
[3 marks]
VDD
R2
+
+
Vo
Vr RI
-
...L
':'
Figure 3.1
Page 4 of7
CONTINUED
ELEC202P
4.
(a) Show briefly how bipolar junction transistors (BITs) are connected to forril the following
circuits:
i) A buffered current mirror. [4 marks]
ii) A fully-differential single-stage amplifier with resistive loads. [4 marks]
(b) .Figure 4.1 shows a BIT amplifier with active load (transistor Q2). For all transistors,
Is = 10 pA and IVAI= 100 V, where Is is the saturation current and VA is the Early voltage.
Let Vcc = 3.3 V and assume that the thermal voltage VT= 26 mV and the current gain
/3=00.
i) Sketch itsDC transfer characteristic (V; versus Vo) and indicate the approximate region
over which the circuit operates as a linear amplifier. [3 marks]
ii) Show its small-signal low-frequency equivalent circuit and derive an expression for the
small-signal voltage gain. [4 marks]·
iii) Determine the value of VEB2such that IREF = 0.5 mA (ignore the Early voltage) and
hence find the value ofRl. [5 marks]
iv) What value of input voltage V; will produce VCEO = VEC2? (do not ignore the Early
voltage). [5 marks]
Vcc
[REF
Figure 4.1
Page 5 of7
TURNOVER
ELEC202P
5.
(a) Define the tenn conversion efficiency as applied to a power amplifier~
[3 marks]
r (b) Explain the statement that 'a power amplifier can be regarded as a frequency converter'.
[3 marks]
(c) Figure 5.1 shows a power amplifier output stage circuit. For a sine-wave input' (VI) the circuit
is required to deliver an average power of 50 W into a 10 n load (RL). The power supply
should be 2 V greater than the corresponding peak sine-wave output voltage.
i) What class of power amplifier does this circuit belong to?
[3 marks]
ii) Detennine the power supply voltage required (to the nearest volt in the appropriate
direction).
. [3 marks]
iii} Detennine the peak current from each supply.
[3 marks]
iv) Detennine the total supply power.
[3 marks]
v) Detennine the power-conversion efficiency.
[3 marks]
vi) Detennine the maximum possible power dissipation in each transistor.
[4 marks]
Vcc
VIo-----e
-Vcc
Figure 5.1
Page 6 of7
CONTINUED
ELEC202P
6.
(a) State the main types of sinusoidal oscillators available noting clearly the approximate
frequency range for each type.
[4 marks]
(b) An amplifier with feedback is used as the basis of a sinusoidal oscillator. Draw the block
diagram of such a system. If the gain of the amplifier is A(s) and the gain of the feedback
path is fJ(s), where s is the Laplace frequency variable, derive the conditions for oscillation
to occur.
[6 marks]
(c) Consider the oscillator in Figure 6.1. The opamp has ideal characteristics.
i) What type of oscillator is this circuit?
[3 marks]
ii) Analyse the circuit to show that the frequency of oscillation is given by
1
OJ =
o RC
[8 marks]
iii) If R = RI = 10 kO, and C = 100 pF calculate the frequency of oscillation and the
required value of R2 in order to sustain oscIllation.
