IFB270 Advanced Electronic Circuits Chapter 9: FET amplifiers and switching circuits Prof. Manar Mohaisen Department of EEC Engineering
Review of the Precedent Lecture Review of basic electronic devices 2
Lecture Objectives Review of the JFET and MOSFET Explain and analyze the operation of common-source, commondrain, and common-gate amplifiers Explain the structure of a cascade amplifier Discuss the operation of a class D amplifier Describe how MOSFETs are used in analog switching applications Describe how MOSFETs are used in digital switching applications 3
JFET Two Types: n channel and p channel Review of JFET The leads are connected to each end of the channel Drain: Connected to the upper end of the channel Source: Connected to the lower end of the channel In case of n channel JFET Two p-type regions are diffused in the n-type material to form a channel. In case of p channel JFET, two n-type regions are used. 4
Review of JFET contd. Zero V GG Ohmic Region When V DS increases, I D will increases proportionally. Active Region Channel resistance is essentially constant. I D becomes essentially constant. Starting at the Pinch-Off Voltage, V p. D y g g, p V GD increases and creates a depletion region that offsets the increase in V DS. Breakdown The point at which I D starts to increase very rapidly with any increase in V DS. 5
Current-voltage Relationship Review of JFET contd. I D 2 VGS GS(off) I 1 GS DSS V 6
More More on FET Input resistance is very high (input current, i.e., gate current, almost equals zero) FETs have a low noise Advantageous for amplifying low-level signals FET amplifiers have the advantage because biasing is simple and more efficient Compared to the BJT amplifiers FET circuits have the disadvantage of low gain and higher distortion FETs have three amplifier configurations Common-source source, common-drain, and common-gate FETs have four power amplifier types A, B, C, and D 7
The MOSFET Metal Oxide Semiconductor FET (MOSFET) The MOSFET does not have a pn junction The gate is insulated from the channel by a silicon dioxide (SiO2) Basic Types of MOSFET Enhancement MOSFET (E-MOSFET) Depletion MOSFET (D-MOSFET) 8
The MOSFET contd. Enhancement MOSFET (E-MOSFET) It does not have structural channel The +ve gate voltage must be above a threshold to induce a channel V DD pulls the electrons and makes the current flow 9
The MOSFET contd. Depletion MOSFEST (D-MOSFET) It has a structural channel. It has two modes of operation (n-channel) Depletion mode: When a negative gate-to-source is applied (General operation mode.) Enhancement mode: When a positive gate-to-source is applied. The p-channel is similar with reversed voltage polarities. 10
The MOSFET contd. Depletion MOSFEST (D-MOSFET) contd. Depletion Mode The gate is a plate and the channel as the other plate of a capacitor. Negative voltage at the gate will deplete electrons from the channel. Therefore, the conductivity of the channel will be reduced. At a sufficiently large gate voltage, V GS(off),, I D is zero. Enhancement Mode Positive voltage at the gate will attract more electrons from the channel. Therefore, the conductivity of the channel will be increased. 11
The Common-source Amplifier Internal FET equivalent circuit Current source equals g m V gs Resistances r gs is assumed to be extremely large (i.e., an open circuit) r ds is assumed to be large enough to neglect 12
The CS Amplifier contd. Voltage gain A v V = out = Vin I R V V ds gs = d d I / g = d m g R m d g m is the transconductance Example gm = 4 ms and Rd = 1.5 kω. What is the ideal voltage gain? A v = g m R d = 6 (very low compared to BJT circuits) 13
CS amplifier Input is applied to the gate The CS Amplifier contd. Output is taken from the drain The source is common for the input and output The resistor R G is for two objectives It keeps the gate at approximately 0V Its high voltage prevents loading the ac voltage source 14
R D C 3 The CS Amplifier contd. CS amplifier contd. Operation for an n-channel JFET Due to the ac input source The output voltage (V ds ) swings above and below its dc value The input voltage (V gs ) swings above and below its dc value Input and output are 180 o out of phase dc analysis ac analysis +V DD V out C 1 R L V in R G R S C 2 15
The CS Amplifier contd. DC analysis of the CS amplifier Find the Q-point, i.e., I DQ and V DSQ Capacitors are replaced with open-circuits JFET CS amplifier DC equivalence 16
The CS Amplifier contd. DC analysis of the CS amplifier contd. Graphical approach Given that I DSS = 4.3 ma and V GS(off) =-77V 7.7 V. V GS = 0.3 V GS(off) (-2.31 V) I D = I DSS /2 (2.15 ma) V GS = 0.5 V GS(off) (-3.85 V) I D = I DSS /4 (1.075 ma) The load line It goes from the origin to the point I D = I DSS and V GS = I DSS R S DC equivalence Mathematical approach 2 D S GS(off) I R I = D IDSS 1 V 17
The CS Amplifier contd. AC analysis of the CS amplifier Replace the capacitors by short-circuits and dc sources by ground A v V V = out = ds Vin Vgs I R = d d = g m R I / g d m d g m m0 = g 1 V V GS GS(off) g m0 2I = DSS V GS(off) R d = R D R L (effect of load on voltage gain) 18
The CS Amplifier contd. Input resistance of the CS amplifier R in = R G V I GS GSS Example: I GSS = 30 na at V GS = 10 V V 10V R GS in = RG = 10M Ω = 9.7M Ω R I GSS 30nA G 19
The CS Amplifier contd. D-MOSFET amplifier operation A zero-biased common-source n-channel D-MOSFET DC analysis V GS = 0, therefore AC analysis VD = VDD IDRD The same as for the JFET amplifier. 20
The CS Amplifier contd. E-MOSFET amplifier operation Using the voltage-divider bias, Thegateisbiasedwitha+ve a voltage such that V GS > V GS(th) V GS = R R 2 1 + R 2 V DD I = K( V V ) D GS GS(th) 2 V = V I R DS DD D D R = R R ( V / I ) in 1 2 GS GSS 21
Example 9-7 The CS Amplifier contd. Determine the peak-to-peak variation in I d when V gs is varied ±1 about the Q-point. Solution: JFET: Q-point is at V GS = -2 V and I D = 2.5 ma, peak-to-peak of change in I d = 1.6 ma D-MOSFET: Q-point is at V GS = 0 V and I D = I DSS = 4 ma, p-t-p of change in I d = 2.8 ma E-MOSFET: Q-point is at V GS = 8 V and I D = 2.5 ma, p-t-p of change in I d = 2.2 ma 22
The CD amplifier The Common-drain Amplifier A CD amplifier is also called a source-follower Voltage gain V I R s A out d v = = Vin Vgs + Id Rs gv m gsrs = Vgs + gmvgsrs = g 1 m R g s + R Input and output are in-phase The input resistance m s ( ) R = R V / I in G GS GSS 23
The Common-gate Amplifier The CG amplifier Voltage gain A v V out V = = d Vin Vgs I R g d d mvgsr = = d = V V gs gs g R m d Input resistance V in Vgs Vgs R 1 in = = = = I I g V g in d m gs m 24
Review of FET Amplifiers Comparison Common-source Common-drain Common-gate A Voltage gain v m d = g R A R v = g 1 m g s + R v m d m s A = g R v m d Input R R in = R G V I GS GSS ( ) R = R V / I in G GS GSS R in = 1 g m 25
Example Cascading CS and CG amplifiers The Cascade Amplifier Increases the input resistance Reduces the internal capacitance to allow for operation at much higher frequencies A = g R = g X v2 m(cs) d m(cs) = g m L (CS) (2 π fl ) A = A A = g X v v1 v2 m(cs) L R in = R 3 V I GS GSS A = g R = g R v1 m(cs) d m(cs) in(cg) = g 1 1 m(cs) g m(cg) 26
Amplifiers The Class D Amplifier Classes A, B, and AB (Chapter 7) can use either BJTs or FETs However, Class D used only MOSFETs Transistors are either switched on or off in response to an analog input 27
The Class D Amplifier contd. Basic class D audio amplifier The complementary MOSFET amplifier Ideally, efficiency = 100 % The output is a rectangular wave that includes the input frequency and (an infinite) a number of sinusoids We need a low-pass filter to remove harmonics 28
The Class D Amplifier contd. Basic class D audio amplifier The low-pass filter removes the unnecessary harmonics and maintains an amplified version of the input signal 29
The Class D Amplifier contd. Basic class D audio amplifier Signal flow in a class D amplifier 30
MOSFET Analog Switching Characteristics of the MOSFET Very low on-resistance, very high off-resistance, and fast switching times E-MOSFETs as switches When V GS is less than V GS(th), the MOSFET is off When V GS is greater than V GS(th), the MOSFET is on 31
MOSFET Analog Switching contd. The analog switch (n-channel MOSFET) The input (at the drain) is connected to the source when V GS greater than the threshold value (MOSFET is on), that is V = V V V GS G p( out) GS(th) When V G is 0, V GS is less than V GS(th) and the MOSFET is off 32
MOSFET Analog Switching contd. Analog Switch Applications Sampling circuit Nyquist frequency: Sampling frequency must be larger than double the maximum input frequency: f sample(min) > 2f signal 33
MOSFET Analog Switching contd. Analog Switch Applications Analog multiplexer The two MOSFETs are alternately turned on and off so that the two signals are alternately taken at the output 34
MOSFET Digital Switching Complementary MOS (CMOS) Fast switching between Q1 and Q2: negligible power dissipation This circuit is referred to as inverter in digital electronics 35
NAND gate MOSFET Digital Switching contd. When V A AND V B are high, the output is low; otherwise, the output is high 36
NOR gate MOSFET Digital Switching contd. When V A or V B or both are high, the output is low; otherwise, the output is high 37
Keywords Keywords Common-source, common-drain, and common gate amplifiers Class D amplifier Analog/digital switch CMOS 38
Lecture Summary Reviewed of the JFET and MOSFET Explained and analyzed the operation of common-source, common-drain, and common-gate amplifiers Explained the structure of a cascade amplifier Discussed the operation of a class D amplifier Described how MOSFETs are used in analog switching applications Described how MOSFETs are used in digital switching applications 39