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We will now investigate the frequency effects introduced by the larger capacitive elements of the network at low frequencies and the smaller capacitive elements of the active device at high frequencies .
FREQUENCY RESPONSE BJT AND FET Introduction We will now investigate the frequency effects introduced by the larger capacitive elements of the network at low frequencies and the smaller capacitive elements of the active device at high frequencies Low, High & Mid Frequency Range Typical Frequency Response The band frequencies define a level where the gain or quantity of interest will be 70.7% of its maximum value • Normalized plot • Decibel plot • Phase plot LOW FREQUENCY ANALYSIS- BODE PLOT Defining the Low Cutoff Frequency In the low-frequency region of the single-stage BJT or FET amplifier, it is the RC combinations formed by the network capacitors CC, CE, and Cs and the network resistive parameters that determine the cutoff frequencies , -1448 Lec#7 , Nov 2014 © Ahmad El Voltage-Divider Bias Config • , J- 601 -1448 • A change in frequency by a factor of two, equivalent to one octave, results in a 6-dB change in the ratio, as shown by the change in gain from fL/2 to fL For a 10:1 change in frequency, equivalent to one decade, there is a 20-dB change in the ratio, as demonstrated between the frequencies of fL/10 and fL The piecewise linear plot of the asymptotes and associated breakpoints is called a Bode plot of the magnitude versus frequency Lec#7 , Nov 2014 © Ahmad El Bode Plot • • Phase Angle: © Ahmad El Loaded BJT Amplifier In the voltage-divider ct => the capacitors Cs, CC , and CE will determine the low-frequency response Cc: , J- 601 -1448 Cs : Lec#7 fL= max(fLs , fLc , fLE) Cs : Impact of RS FroydWess - Online Notes FET Amplifier High-Frequency Response Capacitances that affect the high-frequency response are • Junction capacitances Cgs, C Cgd, ds • Wiring capacitances Cw C i, wo • Coupling capacitors CG, CC • Bypass capacitor CS Figure 11.52 Capacitive elements that affect the high frequency response of a JFET amplifier FroydWess - Online Notes Input Network (fHi) High-Frequency Cutoff f Hi = 2πR Thi Ci Ci C + Cgs + = Wi CMi C = (1 − A Mi = vR )Csigd || R g RG Thi FroydWess - Online Notes Output Network (fHo) High-Frequency Cutoff f Ho = 2πR Tho Co Co C + Cds + = Wo C⎞Mo ⎛ =⎜ ⎜ ⎟Cg ⎝− ⎟A v ⎠d R = || R || Tho R D L rd C Mo FroydWess - Online Notes Example: FroydWess - Online Notes Solution: FroydWess - Online Notes FroydWess - Online Notes Multistage Frequency Effects Each stage will have its own frequency response, but the output of one stage will be affected by capacitances in the subsequent stage This is especially so when determining the high frequency response For example, the output capacitance (Co) will be affected by the input Miller Capacitance (CMi) of the next stage FroydWess - Online Notes Multistage Amplifier Frequency Response Once the cutoff frequencies have been determined for each stage (taking into account the shared capacitances), they can be plotted Note the highest lower cutoff frequency (fL) and the lowest upper cutoff frequency (fH) are closest to the actual response of the amplifier FroydWess - Online Notes Square Wave Testing In order to determine the frequency response of an amplifier by experimentation, you must apply a wide range of frequencies to the amplifier One way to accomplish this is to apply a square wave A square wave consists of multiple frequencies (by Fourier analysis: it consists of odd harmonics) FroydWess - Online Notes Square Wave Response Waveforms If the output of the amplifier is not a perfect square wave then the amplifier is ‘cutting’ off certain frequency components of the square wave FroydWess - Online Notes Example: FroydWess - Online Notes Solution: FroydWess - Online Notes Follow me! Online Notes and Presentations Visit: wwww.FroydWess.com Homework: Poblems Page 539-543: 2, 4, 8, 15, 16, 19, 22, 26, 28, 31 FroydWess - Online Notes [...]... ∞Ω FroydWess - Online Notes FET Amplifier Low -Frequency Response The Bode plot indicates that each capacitor may have a different cutoff frequency The capacitor that has the highest lower cutoff frequency (fL) is closest to the actual cutoff frequency of the amplifier FroydWess - Online Notes BJT Amplifier High -Frequency Response Capacitances that affect the high -frequency response are • Junction capacitances... varies with frequency β f ≅ FroydWess - Online Notes 2πβmid1r e (Cbe + Cbc ) BJT Amplifier Frequency Response Note the highest lower cutoff frequency (fL) and the lowest upper cutoff frequency (fH) are closest to the actual response of the amplifier.FroydWess - Online Notes Example: FroydWess - Online Notes Solution: FroydWess - Online Notes FroydWess - Online Notes FET Amplifier High -Frequency Response. .. J- 601 -1448 Network Parameters FET Amplifier Cutoff Frequencies: The mid-range frequency range of an amplifier is called the bandwidth of the amplifier The bandwidth is defined by the lower and upper cutoff frequencies Cutoff – any frequency at which the gain has dropped by 3 dB BJT Amplifier Low -Frequency Response: At low frequencies, coupling capacitor (CS, CC) and bypass capacitor (CE) reactances... the circuit impedances Figure 11.16 Loaded BJT amplifier with capacitors that affect the low -frequency response FroydWess - Online Notes BJT Amplifier Low -Frequency Response The Bode plot indicates that each capacitor may have a different cutoff frequency It is the device that has the highest lower cutoff frequency (fL) that dominates the overall frequency response of the amplifier FroydWess - Online... Network of Fig 11.16 with the capacitors that affect the high -frequency response FroydWess - Online Notes Input Network (fHi) High -Frequency Cutoff f Hi = 1 2πR Thi Ci where RT =Rs || R1 || hi || R 2 Ri and Ci = CWi + Cbe + CMi = CWi + Cbe + (1 − A v )Cbc FroydWess - Online Notes Output Network (fHo) High -Frequency Cutoff f Ho = 1 2πR Tho Co where R and = RC || R Tho L || ro Co C + Cce + = Wo CMo FroydWess... Plot A Bode plot indicates the frequency response of an amplifier The horizontal scale indicates the frequency (in Hz) and the vertical scale indicates the gain (in dB) Figure 11.4 Gain versus frequency: (a) RCcoupled amplifiers; (b) transformercoupled amplifiers; (c) direct-coupled amplifiers FroydWess - Online Notes à à Lec#7 , Nov 2014 © Ahmad El à , J- 601 -1448 FET Amplifier capacitive elements... High -Frequency Response Capacitances that affect the high -frequency response are • Junction capacitances Cgs, C Cgd, ds • Wiring capacitances Cw C i, wo • Coupling capacitors CG, CC • Bypass capacitor CS Figure 11.52 Capacitive elements that affect the high frequency response of a JFET amplifier FroydWess - Online Notes Input Network (fHi) High -Frequency Cutoff f Hi = 1 2πR Thi Ci Ci C + Cgs + = Wi CMi... internal to the active device and the wiring capacitance between leads of the network - For any inverting amplifier, the input capacitance will be increased by a Miller effect capacitance sensitive to the gain of the amplifier and the interelectrode (parasitic) capacitance between the input and output terminals of the active device Lec#7 , Nov 2014 © Ahmad El - In the high -frequency region, the , J- 601... negative capacitance (for Av > 1) For noninverting amplifiers such as the common-base and emitter-follower configurations, the Miller effect capacitance is not a contributing concern for high -frequency applications The Miller effect will also increase the level of output capacitance, which must also be considered when the high -frequency cutoff is determined Lec#7 , Nov 2014 © Ahmad El • • , J- 601 -1448 Miller... dB loss-per-decade Coupling Capacitor (CG) The cutoff frequency due to CG can be calculated with f LC = 1 2π(R sig + R i)C G where Ri R = G FroydWess - Online Notes Coupling Capacitor (CC) The cutoff frequency due to CC can be calculated with f LC = 1 2π(R o+ R L )CC where Ro = R D || rd FroydWess - Online Notes Bypass Capacitor (CS) The cutoff frequency due to CS can be calculated with f LS = 1 2πR