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Electronic Devices Ninth Edition Floyd Chapter 9 Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary The Common-Source Amplifier In a CS amplifier, the input signal is applied to the gate and the output signal is taken from the drain. The amplifier has higher input resistance and lower gain than the equivalent CE amplifier. +VDD RD C3 Vout C1 RL Vin RG RS C2 The voltage gain is given by the equation Av = gmRd. Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary The Common-Source Amplifier Recall that conductance is the reciprocal of resistance and admittance is the reciprocal of impedance. Data sheets typically specify the forward transfer admittance, yfs rather than transconductance, gm. The definition of yfs is I y fs DYNAMIC CHARACTERISTICS Forward Transfer Admittance (VDS = 15 Vdc, VGS = 0) D VG Symbol 2N5457 2N5458 |Yfs| Min Typ 1000 1500 3000 4000 Max 5000 5500 Unit m mhos An alternate gain expression for a CS amplifier is Av = yfsRd. Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary The Common-Source Amplifier You can estimate what the transfer characteristic looks like from values on the specification sheet, but keep in mind that large variations are common with JFETs. For example, the range of specified values for a 2N5458 is shown. OFF CHARACTERISTICS Gate-Source Cutoff Voltage (VDS = 15 Vdc, iD = 10 nAdc) Electronic Devices, 9th edition Thomas L. Floyd 9 2 – VGS (V) Symbol 2N5457 2N5458 ON CHARACTERISTICS Zero Gate-Source Drain Current (VDS = 15 Vdc, VGS = 0) ID (mA) 2N5457 2N5458 V GS(off) Min -0.5 -1.0 –7 –1 Typ - Max -6.0 -7.0 Symbol Min Typ Max I DSS 1.0 2.0 3.0 6.0 5.0 9.0 0 Unit Vdc Unit mAdc © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary The Common-Source Amplifier To analyze the CS amplifier. you need to start with dc values. It is useful to estimate ID based on typical values; specific circuits will vary from this estimate. VDD +12 V For a typical 2N5458, what is the drain current? RD 2.7 kW Vout C1 2N5458 From the specification sheet, the typical IDSS = 6.0 mA and VGS(off) = -4 V. These values can be plotted along with the load line to obtain a graphical solution. Electronic Devices, 9th edition Thomas L. Floyd 0.1 mF Vin 100 mV RG 10 MW RS 470 W C2 10 mF See the following slide… © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary The Common-Source Amplifier (continued) A graphical solution is illustrated. On the transconductance curve, plot the load line for the source resistor. Load line for 470 W resistor ID (mA) 6 Then read the current and voltage at the Q-point. ID = 2.8 mA and VGS = -1.3 V Electronic Devices, 9th edition Thomas L. Floyd Q – VGS (V) –4 -1.3 V 2.8 mA 0 © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary The Common-Source Amplifier (continued) I D RS Alternatively, you can obtain ID using Equation 9-2: I D I DSS 1 V GS(off) 2 The solution to this quadratic equation is simplified using a calculator that can handle quadratic equations. ID=IDSS (1–(–ID RS/VG... After entering the equation, enter the ID= .0027494671581759 IDSS= .006 known values, but leave ID open. RS= 470 For the typical values for the 2N5458, enter absolute VGSOFF= 4.0 value (IDSS = 6 mA and VGS(off) = -4 V) with bound=(–1E 99,1E 99) a source resistance of 470 W, we find GRAPH RANGE ZOOM TRACE SOLVE 2.75 mA. press F5 Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary The Common-Source Amplifier Assume IDSS is 6.0 mA, VGS(off) is -4 V, and VGS = -1.3 V as found previously. What is the expected gain? Output is inverted gm0 VDD +12 V 2 6.0 mA 2 I DSS 3.0 mS 4 V VGS(off) V g m g m 0 1 - GS V GS(off) -1.3 V 3.0 mS 1 -4.0 V 2.02 mS RD 2.7 kW Vout C1 2N5458 0.1 mF Vin 100 mV RG 10 MW RS 470 W C2 10 mF Av = gmRd = (2.02 mS)(2.7 kW) = 5.45 Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary The Common-Source Amplifier The gain is reduced when a load is connected to the amplifier because the total ac drain resistance (Rd) is reduced. VDD +12 V How does the addition of the 10 kW load affect the gain? RD 2.7 kW Rd RD RL RD RL 2.7 kW 10 kW 2.7 kW 10 kW 2.13 kW Vout C1 2N5458 0.1 mF Vin 100 mV RG 10 MW RS 470 W C2 10 mF RL 10 kW Av = gmRd = (2.02 mS)(2.13 kW) = 4.29 Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary The D-MOSFET In operation, the D-MOSFET has the unique property in that it can be operated with zero bias, allowing the signal to swing above and below ground. This means that it can operate in either D-mode or E-mode. En ha nc em ent ID +VDD Q RD C2 Vout tio le p e D C1 RL Vin –VGS n Id 0 +VGS RG Vgs Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary The E-MOSFET The E-MOSFET is a normally off device. The n-channel device is biased on by making the gate positive with respect to the source. A voltagedivider biased E-MOSFET amplifier is shown. ID Enhancement +VDD RD R1 C3 Vout C1 Id RL Vin Q IDQ R2 RS C2 0 VGS VGS(th) Vgs VGSQ Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary The E-MOSFET The E-MOSFET amplifier in Example 9-8 is illustrated in Multisim using a 2N7000 MOSFET. Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary The Common-Drain Amplifier In a CD amplifier, the input signal is applied to the gate and the output signal is taken from the source. There is no drain resistor, because it is common to the input and output signals. +VDD C1 Vin C2 RG RS Vout RL g m Rs The voltage gain is given by the equation Av 1 g m Rs The voltage gain is always < 1, but the power gain is not. Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary The Cascode Amplifier The cascode connection is a combination of CS and CG amplifiers. This forms a good high-frequency amplifier. The input and output signals at 10 MHz are shown for this circuit on the following slide… Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary The Cascode Amplifier The input signal for the cascode amplifier is shown in red; the output is blue. What is the gain? The peak of the input is 24.7 mV. The peak of the output is 2.33 V. AV = 94.3 Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary The Class-D Amplifier MOSFETs are useful as class-D amplifiers, which are very efficient because they operate as switching amplifiers. They use pulse width modulation, a process in which the input signal is converted to a series of pulses. The pulse width varies proportionally to the amplitude of the input signal. Pulse-width modulation is easy to set up in Multisim. The following slide shows the circuit. A sine wave is compared to a faster triangle wave of the about the same amplitude using a comparator (a 741 op-amp can be used at low frequencies). Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary The Class-D Amplifier A circuit that you can use in lab or in Multisim to observe pulse width modulation in action. The scope display is shown on the following slide… Electronic Devices, 9th edition Thomas L. Floyd Op-amp set up as a comparator © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary The Class-D Amplifier The signal is the yellow sine wave and is compared repeatedly to the triangle (cyan). The result of the comparison is the output (magenta). Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary The Class-D Amplifier The modulated signal is amplified by class-B complementary MOSFET transistors. The output is filtered by a low-pass filter to recover the original signal and remove the higher modulation frequency. PWM is also useful in control applications such as motor controllers. MOSFETs are widely used in these applications because of fast switching time and low onstate resistance. Electronic Devices, 9th edition Thomas L. Floyd +VDD Q1 Modulated input Low-pass filter RL Q2 –VDD © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Summary The Analog Switch MOSFETs are also used as analog switches to connect or disconnect an analog signal. Analog switches are available in IC form – for example the CD4066 is a quad analog switch that used parallel n- and p-channel MOSFETs. The configuration shown allows signals to be passed in either direction. Advantages of MOSFETs are that they have relatively low on-state resistance and they can be used at high frequencies, such as found in video applications. Electronic Devices, 9th edition Thomas L. Floyd IN/OUT OUT/IN Control Simplified internal construction of a bidirectional IC analog switch. © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Selected Key Terms Common-source A FET amplifier configuration in which the source is the (ac) grounded terminal. Common-drain A FET amplifier configuration in which the drain is the (ac) grounded terminal. Source-follower The common-drain amplifier. Class-D A nonlinear amplifier in which the transistors amplifier are operated as switches. Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Selected Key Terms Pulse-width A process in which a signal is converted to a modulation series of pulses with widths that vary proportionally to the signal amplitude. Analog switch A device that switches an analog signal on and off. CMOS Complementary MOS. Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Quiz 1. Compared to a common-emitter amplifier, a commonsource amplifier generally will have a. higher gain and higher input resistance b. higher gain and lower input resistance c. lower gain and higher input resistance d. lower gain and lower input resistance Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Quiz 2. The abbreviation yfs means a. forward transfer admittance b. forward on-state resistance c. reverse transfer susceptance d. reverse on-state conductance Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Quiz 3. The plot shown is a graphical solution for a self-biased FET amplifier. The red line represents the a. gate resistor ID (mA) b. source resistor c. drain resistor 6 d. none of the above – VGS (V) Electronic Devices, 9th edition Thomas L. Floyd –4 0 © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Quiz 4. The resistance represented by the red line is a. 150 W b. 240 W ID (mA) c. 470 W d. 666 W 6 – VGS (V) Electronic Devices, 9th edition Thomas L. Floyd –4 0 © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Quiz g m Rs 5. The gain equation Av is used to calculate the 1 g m Rs gain of a. a CS amplifier b. a CD amplifier c. a CG amplifier d. any of the above Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Quiz 6. A FET that can be biased with zero bias is a a. an n-channel JFET b. a D-MOSFET c. an E-MOSFET d. all of the above Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Quiz 7. The cascode amplifier shown uses a. A CS and a CD stage b. Two CS stages c. Two CD stages d. none of the above Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Quiz 8. The principle circuit used in creating a pulse width modulator is a a. peak detector b. clipper c. comparator d. low-pass filter Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Quiz 9. The circuit is an amplifier for a pulse width modulated signal. The load has the demodulated signal. The yellow box represents a +V DD a. peak detector Q1 b. clipper c. comparator d. low-pass filter Modulated input RL Q2 –VDD Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Quiz 10. When the control signal is active, the output of an analog switch should look like a. the input signal b. a square wave c. a modulated pulse d. a dc level Electronic Devices, 9th edition Thomas L. Floyd © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved. Quiz Answers: Electronic Devices, 9th edition Thomas L. Floyd 1. c 6. b 2. a 7. d 3. b 8. c 4. d 9. d 5. b 10. a © 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.