ET 212 Electronics Amplifier Frequency Response Electrical and Telecommunication Engineering Technology Professor Jang Acknowledgement I want to express my gratitude to Prentice Hall giving me the permission to use instructors material for developing this module. I would like to thank the Department of Electrical and Telecommunications Engineering Technology of NYCCT for giving me support to commence and complete this module. I hope this module is helpful to enhance our students academic performance. Outlines Introduction to Frequency Response of an Amplifier Gain of an Amplifier in Decibels (dB) Frequency Response of a BJT Amplifier

Frequency Response of an FET Amplifier Frequency Response of a Multistage Amplifier Key Words: Frequency Response, Amplifier, Decibel, BJT, FET, Multistage ET212 Electronics Amplifier Frequency Response Floyd 2 Amplifier Frequency Response Introduction Most amplifiers have a finite range of frequencies in which it will operate. We will discuss what determines the frequency response of an amplifier circuit and how it is measured. ET212 Electronics Amplifier Frequency Response Floyd 3

Basic Concepts When frequency is low enough, the coupling and bypass capacitors can no longer be considered as shorts because their reactances are large enough to have significant effect. Also, when the frequency is high enough, the internal transistor capacitances can no longer be considered as opens because their reactances become small enough to have significant effect on the amplifier operation. We will discuss how the capacitor limits the passage of certain frequencies. This is called the frequency response of an amplifier. ET212 Electronics Amplifier Frequency Response Floyd 4 Basic Concepts Effect of Coupling Capacitors Coupling capacitors C1 and C3 limit the passage of very low frequencies. Emitter bypass C2 capacitor will have high reactance to low frequencies as well, limiting the gain. Also the capacitance causes a phase shift of the signal.

1 XC 2fC ET212 Electronics Amplifier Frequency Response Floyd 5 Basic Concepts Effect of Bypass Capacitors At lower frequencies, the reactance of the emitter bypass capacitor, C2 in previous Figure, becomes significant and emitter is no longer at ac ground. The capacitive reactance XC2 in parallel with RE creates an impedance that reduces the gain as shown in Figure. When the frequency is sufficiently high XC 0 and the voltage gain of the CE amplifier is RC Av

r 'e At lower frequencies, XC >> 0 and the voltage gain is RC Nonzero reactance of the bypass capacitor in parallel with RE creates an emitter impedance, (Ze), which reduces the voltage gain. Av r ' Z e Floyd e 6 Basic Concepts Effect of Internal Transistor Capacitances

At high frequencies, the coupling and bypass capacitors become effective ac shorts and do not affect an amplifiers response. Internal transistor junction capacitances, however, do come into play, reducing an amplifiers gain and introducing phase shift as the signal frequency increases. Cbe is the base-emitter junction capacitance and C is the base-collector junction capacitance. 7 Basic Concepts Effect of Internal Transistor Capacitances When the reactance of Cbe becomes small enough, a significant amount of the signal voltage is lost due to a voltage-divider effect of the signal source resistance and the reactance of Cbe as illustrated in Figure (a). When the resistance of Cbc becomes small enough, a significant amount of output signal voltage is fed back out of phase with input (negative feedback), thus effectively reducing the voltage gain as shown in Figure (b). ET212 Electronics Amplifier Frequency Response Floyd

8 Basic Concepts Millers Theorem At high frequencies, the coupling and bypass capacitors become effective ac shorts and do not affect an amplifiers response. Internal transistor junction capacitances, however, do come into play, reducing an amplifiers gain and introducing phase shift as the signal frequency increases. Millers theorem states that C effectively appears as a capacitance from input and output to ground, as shown in Figure (b). Cin(Miller) = C(Av + 1) ET212 Electronics Amplifier Frequency Response Cout ( Miller ) Av 1 C Av Floyd

9 Basic Concepts Millers Theorem Millers theorem allows us to view the internal capacitances as external capacitors for better understanding of the effect they have on the frequency response. ET212 Electronics Amplifier Frequency Response Floyd 10 The Decibel The decibel is a common unit of measurement of voltage gain and frequency response. It is a logarithmic measurement of the ratio of one power to another or one voltage to another. The formulas below are used for calculation of decibels for power gain and voltage gain. Ap(db) = 10 log Ap Av(db) = 20 log Av

ET212 Electronics Amplifier Frequency Response Floyd 11 Table shows how doubling or having voltage gains translates into dB values. Notice in the table that every time the voltage gain is doubled, the dB value increases by 6 dB, and every time the gain is halved, the dB value decreases by 6 dB. VOLTAGE GAIN (Av) dB (WITH RESPECT TO ZERO REFERENCE) 32 20 log(32) = 30 dB 16 20 log(16) = 24 dB 8

20 log(8) = 18 dB 4 20 log(4) = 12 dB 2 20 log(2) = 6 dB 1 20 log(1) = 0 dB 0.707 20 log(0.707) = - 3 dB 0.5 20 log(0.5) = - 6 dB

0.25 20 log(0.25) = - 12 dB 0.125 20 log(0.125) = - 18 dB 0.0625 20 log(0.0625) = - 24 dB 0.03125 20 log(0.03125) = - 30 dB ET212 Electronics Amplifier Frequency Response Floyd 12 Ex 10-1 Express each of the following ratios in dB:

Pout Pout 250 100 (a) (b) (c) Av = 10 Pin Pin (d) Ap = 0.5 (e) Vout 0.707 Vin (a) Ap(dB) = 10 log(250) = 24 dB (b) Ap(dB) = 10 log(100) = 20 dB (c) Av(dB) = 20 log(10) = 20 dB (d) Ap(dB) = 10 log(0.5) = - 3 dB (e) Av(dB) = 20 log(0.707) = - 3 dB

ET212 Electronics Amplifier Frequency Response Floyd 13 The Decibel The Critical Frequency The critical frequency also known as the cutoff frequency is the frequency at which the output power drops by 3 dB, which represents one-half of its midrange value. An output voltage drop of 3 dB represents about a 70.7% drop from the midrange value. Power is often measured in units of dBm. This is decibels with reference to 1mW of power. This means that 0 dBm = 1mW. ET212 Electronics Amplifier Frequency Response Floyd 14

Ex 10-2 A certain amplifier has a midrange rms output voltage of 10 V. What is the rms output voltage for each of the following dB gain reductions with a constant rms input voltage? (a) 3 dB (b) 6 dB (c) 12 dB (d) 24 dB Multiply the midrange output voltage by the voltage gain corresponding to the specified dB value in Table. (a) (b) (c) (d) At 3 dB, Vout = 0.707(10 V) = 7.07 V At 6 dB, Vout = 0.5(10 V) = 5 V At 12 dB, Vout = 0.25(10 V) = 2.5 V At 24 dB, Vout = 0.0625(10 V) = 0.625 V ET212 Electronics Amplifier Frequency Response Floyd 15

Low-Frequency Amplifier Response In looking at the low frequency ac equivalent circuit of a capacitor coupled amplifier we can see there are three RC circuits which will limit low frequency response. The input at the base, the output at the collector, and the emitter. Av ( mid ) A capacitively coupled amplifier. ET212 Electronics Amplifier Frequency Response Rc r 'e The low-frequency ac equivalent circuit of the amplifier in Figure (left) consists of three high-pass RC circuits. Floyd 16

Low-Frequency Amplifier Response The Input RC Circuit The input RC circuit for the BJT amplifier is formed by C1 and the amplifiers input resistance and is shown in Figure. The input circuits effects on the signal at a given frequency can be more easily understood by looking at this simplified input circuit. The frequency at which the gain is down by 3dB is called the lower critical frequency (fc). This frequency can be determined by the formula below. 1 X C1 Rin 2f cC1 1 fc

2RinC1 Vbase Rin Rin2 X C2 1 Vin Input RC circuit formed by the input coupling capacitor and the amplifiers input resistance. 1 fc 2 ( Rs Rin )C1

17 Ex 10-3 For an input RC circuit in a certain amplifier, Rin = 1.0 k and C1 = 1 F. Neglect the source resistance. (a) Determine the lower critical frequency. (b) What is the attenuation of the input RC circuit at the lower critical frequency? (c) If the midrange voltage gain of the amplifier is 100, what is the gain at the lower critical frequency? (a) 1 1 fc 159 Hz 2RinC1 2 (1.0k)(1F ) (b) At fc, Xc1 = Rin. Therefore

Vbase Attenuation 0.707 Vin (c) Av = 0.707 Av(mid) = 0.707(100) = 70.7 ET212 Electronics Amplifier Frequency Response Floyd 18 Low-Frequency Amplifier Response Voltage gain roll-off at low frequency The decrease in voltage gain with frequency is called roll-off. Lets take a frequency that is one-tenth of the critical frequency (f =0.1fc). Since Xc1 = Rin at fc, then Xc1 = 10 Rin at 0.1fc because of the inverse relationship of XC1 and fc. The attenuation of the input RC circuit is, therefore, V R R Attenuation base 2 in 2 2 in Vin

Rin X C 1 Rin (10 Rin ) 2 Rin Rin Rin2 100 Rin2 Rin2 (1 100) Rin 1 1 0.1 Rin 101 101 10 The dB attenuation is Vbase 20 log(0.1) 20dB

20 log Vin ET212 Electronics Amplifier Frequency Response Floyd 19 Low-Frequency Amplifier Response dB/decade The decrease in voltage gain with frequency is called the roll-off. A ten times change in frequency is called a decade. The attenuation measured in dB at each decade is is the dB/ decade. This typical dB Av vs frequency illustrates the relationship. Sometimes rolloff is expressed in dB/octave, which is a doubling or halving of a the frequency.

ET212 Electronics Amplifier Frequency Response dB voltage gain versus frequency for the input RC circuit. Floyd 20 Ex 10-4 The midrange voltage gain if a certain amplifier is 100. The input RC circuit has a lower critical frequency of 1 kHz. Determine the actual voltage gain at f = 1 kHz, f = 100 Hz, and f = 10 Hz. When f = 1 kHz, the voltage gain is 3 dB less than at midrange. At 3 dB, the voltage gain is reduced by a factor of 0.707. Av = (0.707)(100) = 70.7 When f = 100 Hz = 0.1fc, the voltage gain is 20 dB less than at fc. The voltage gain at 20 dB is one-tenth of that at the midrange frequencies. Av = (0.1)(100) = 10 When f = 10 Hz = 0.01fc, the voltage gain is 20 dB less than at f = 0.1fc or 40 dB. The voltage gain at 40 dB is one-tenth of that at 20 dB or one-hundredth that at the midrange frequencies. Av = (0.01)(100) = 1 ET212 Electronics Amplifier Frequency Response

Floyd 21 Low-Frequency Amplifier Response Phase shift in the input RC circuit In addition to reducing the voltage gain, the input RC circuit also causes an increasing phase shift through an amplifier as the frequency decreases. X C1 tan Rin 1 For midrange frequencies, Xc1 0 , so 0 tan 1 (0) 0o tan Rin 1

At the critical frequency, Xc1 = Rin, so Phase angle versus frequency for the input RC circuit. Rin tan tan 1 (1) 45o Rin 1 A decade below the critical frequency, Xc1 = 10Rin, so 10 Rin tan 1 (10) 84.3o Input RC circuit causes the base voltage to lead the input voltage tan below midrange by an amount equal to the circuit phase angle. Rin 1 Low-Frequency Amplifier Response The Output RC Circuit

The output RC circuit affects the response similarly to the input RC circuit. The formula below is used to determine the cutoff frequency of the output circuit. fc = 1/2(RC + RL)C3 Development of the equivalent low-frequency output RC circuit. ET212 Electronics Amplifier Frequency Response Floyd 23 Low-Frequency Amplifier Response- The Bypass RC Circuit The bypass RC circuit is no different in its effect on the gain at low frequencies. For midrange frequencies it is assumed that XC2 0 , effectively shorting the emitter to ground so that the amplifier gain is Rc/re. As frequency is reduced, XC2 increases. The impedance from emitter to ground increases, gain decreases. Av = Rc / (re + Re) ET212 Electronics Amplifier Frequency Response Floyd

24 Low-Frequency Amplifier Response- The Bode Plot A plot of dB voltage gain versus frequency on semilog paper (logarithmic horizontal axis scale and a linear vertical axis scale) is called a Bode plot. A generalized Bode plot for an RC circuit like that shown in Figure (a) appears in part (b) of the figure. An RC circuit and its low-frequency response. (Blue is ideal; red is actual.) 25 High-Frequency Amplifier Response A high-frequency ac equivalent circuit for the BJT amplifier in Figure. Notice that the coupling and bypass capacitors are treated as effective shorts and do not appear in the equivalent circuit. The internal capacitances, Cbe and Cbc, which are significant only at high frequencies, do appear in the diagram. Capacitively coupled amplifier and its high-frequency equivalent circuit. ET212 Electronics Amplifier Frequency Response

Floyd 26 High-Frequency Amplifier Response Millers Theorem in High-Frequency Analysis Looking in from the signal source, the capacitance C appears in the bc Miller input capacitance from base to ground. Cin(Miller) = Cbc(Av + 1) Cbe simply appears as a capacitance to ac ground, as shown in Figure, in parallel with Cin(Miller). Looking in at collector, Cbc appears in the Miller output capacitance from collector to ground. As shown in Figure. Coutput ( Miller ) Av 1 Cbc Av

High-frequency equivalent circuit after applying Millers theorem. ET212 Electronics Amplifier Frequency Response Floyd 27 Low-Frequency Amplifier Response- The Input RC Circuit At high frequencies, the input circuit is as shown in Figure (a), where ac re is the input resistance. By combining Cbe and Cin(Miller) in parallel and repositioning shown in Figure (b). By thevenizing the circuit to left of capacitor, as indicated, the input RC circuit is reduced to the equivalent form shown in Figure (c). XCtot = Rs||R1||R2||ac re Therefore, 1/(2fc Ctot) = Rs||R1||R2||acre and fc = 1/(2(Rs||R1||R2||acre )Ctot Where Rs is the resistance of the signal source and Ctot = Cbe + Cin(miller).

ET212 Electronics Amplifier Frequency Response Floyd 28 Ex 10-10 Derive the input RC circuit for the BJT amplifier in Figure. Also determine the critical frequency. The transistors data sheet provides the following: ac = 125, Cbe = 20 pF, and Cbc = 2.4 pF. First, find re as follows: R2 4.7 k VB VCC 10V 1.76V 26.7 k R1 R2 VE VB 0.7V 1.06V VE 1.06V IE 2.26mA

RE 470 25mV r 'e 11 .1 IE ET212 Electronics Amplifier Frequency Response Floyd 29 The total resistance of the input circuit is Rin(tot) = Rs||R1||R2||ac re = 600 ||22 k||4.7 k||125(11.1 ) = 378 Next, in order to determine the capacitance, you must calculate the midrange gain of the amplifier so that you can apply Millers theorem. Av ( mid ) Rc RC // RL 1.1k 99 r 'e

r 'e 11 .1 Apply Millers theorem. Cin(Miller) = Cbc(Av(mid) + 1) = (2.4 pF)(100) = 240 pF The total input capacitance is Cin(Miller) in parallel with Cbe. Cin(tot) = Cin(Miller) + Cbe = 240 pF + 20 pF = 260 pF fc 1 2 ( Rin ( tot ) )(Cin ( tot ) ) 1 2 (378)(260 pF ) 1.62MHz 30 Total Amplifier Frequency Response Figure (b) shows a generalized ideal response curve (Bode plot) for the BJT amplifier shown in Figure (a). The three break points at the lower critical frequencies (fc1, fc2, and fc3) are produced by the three low-frequency RC

circuits formed by the coupling and bypass capacitors. The break points at the upper critical frequencies, fc4 and fc5, are produced by the two highfrequency RC circuit formed by the transistors internal capacitances. ET212 Electronics Amplifier Frequency Response Floyd 31 Total Amplifier Frequency Response Bandwidth An amplifier normally operates with signal frequencies between fcl and fcu. The range (band) of frequencies lying between fcl and fcu is defined as the bandwidth of the amplifier, as illustrated in Figure. The amplifiers bandwidth is expressed in units of hertz as BW = fcu fcl ET212 Electronics Amplifier Frequency Response Floyd 32