電子電路基礎(chǔ)英文課件：ch5 Frequency Response

1、 The analysis limited to a particular frequency Investigate the frequency effects Larger capacitive elements of the network at low frequency Smaller capacitive elements of the active device at high frequency The concept of the decibel Similar between BJT and FETAt frequencies above and below the mid

2、range, capacitance and any inductance will affect the gain of the amplifier.At low frequencies the coupling and bypass capacitors lower the gain.At high frequencies stray capacitances associated with the active device lower the gain.Also, cascading amplifiers limits the gain at high and low frequenc

3、ies.The of an amplifier refers to the frequency range in which the amplifier will operate with negligible effects from capacitors and capacitance in devices. This range of frequencies can be called the .A mid frequency22ovmidiomidooVAVPRRA half-power frequency fH , fL220.50.7070.5oHPFomidvmidivmidio

4、HPFooPPAVAVPRRBand width (BW)=fH -fLA decibel definition1020logvvdBvmidvmidAAAAThe mid-range frequency of an amplifier is called the bandwidth of the amplifier. The is defined by the lower and upper cutoff frequencies.frequency at which the gain has dropped by:0.5 power0.707 voltage-3dBCorner, band,

5、 break, half-power frequency(a)If the input signal is low frequency, the reactance XC of capacitor is very large. The output signal equals to the input signal approximately.(b)If the frequency is very high, the reactance of capacitor is shorted, the output is also zero.For the case where XC=R the ou

6、tput is 0.707 of the input at the frequency for XC=R. The frequency is determined fromFor the case where XC=RThe output is 3dB drop of the input at the frequency for XC=R. The high frequency cutoff point is determined fromThe gain of any frequency can beIn the magnitude and phase formThe frequency r

7、esponse of low-pass circuit Low frequency, The reactance XC is very large. The output is zero. High frequency, The reactance is shorted, The output equals to the input. Low frequency, The reactance XC is very large. The output is zero. High frequency, The reactance is shorted, The output equals to t

8、he input. For the case where XC=RThe output is 3dB drop of the input at the frequency for XC=R. The high frequency cutoff point is determined fromThe gain of any frequency can beIn the magnitude and phase formThe frequency response of high-pass circuit In general, the horizontal scale is a logarithm

9、ic scale to permit a plot extending from the low to the high frequency regions. If the frequency response is plotted by logarithmic scale, the plot is called the Bode plot A straight line is drawn for the condition 0dB for But the straight line are only accurate for , when there is a 3-dB drop from

10、the midband levelHffHffHff A straight line is drawn for the condition 0dB for But the straight line are only accurate for , when there is a 3-dB drop from the midband levelLffLffLff find the appropriate equivalent resistance for the high-pass circuit the drop in gain at the low frequency is due to t

11、he increasing reactance of CS, CE and CC consider the large capacitor independentlyAC equivalent with re modelThe input is applied to the baseThe output is from the collectorHigh input impedanceLow output impedanceHigh voltage and current gainPhase shift between input and output is 180 CS Assuming t

12、hat CE and CC are shorted, the CS is the only capacitor that will affect the low frequency response. Connected between the applied source and the active device, the effective resistance of the high-pass circuit The Ri is : The cutoff frequency: CC Assuming that CE and CS are shorted, the CC is the o

13、nly capacitor that will affect the low frequency response. Connected between the the active device and applied load, Vi=0V, determine the output resistance Ro with Thevenin equivalent circuit CE Assuming that CC and CS are shorted, the CE is the only capacitor that will affect the low frequency resp

14、onse. 1. Equivalent model for high frequency response - hybrid model rb: base contact, bulk, and spreading resistance. r, ru, ro : resistance between terminalsCbc Cbe : the interelectrode capacitances. r :base-to- emitter resistance is re.For the inverting amplifier, there is Miller effect capacitance. The capacitance of input and output is the function of Cbc 1MivbcCA C11MobcvCCAAt the input terminalAt the output terminalThe capacitor for high frequency responseThe capacitor for high frequency responseThe ac equivalent circuit of high f