Chapter 14 Amplifier Frequency Response Bandwidth n n

Chapter 14 Amplifier Frequency Response

Bandwidth n n Bandwidth (BW) – The range of frequencies over which gain is relatively constant. Frequency response curve – A curve showing the relationship between gain and operating frequency.

Cutoff Frequencies n Cutoff frequencies – The frequencies at which Ap decreases to 50% of its midband value.

Geometric Center Frequency n n Geometric center frequency (f 0) – The frequency that equals the geometric average of f. C 1 and f. C 2. Power gain is maximum when an amplifier is operated at its geometric center frequency.

Gain & Frequency Measurements n n n The y-axis of the graph represents changes in power gain (rather than specific gain values). The changes in gain are represented in d. B. The frequency increments are logarithmic; that is, each increment is some whole-number multiple of the previous increment.

d. B Power Gain Example: Convert Ap = 1200 to d. B form.

d. B Gains Are Additive

Frequency Intervals n n Octave – A frequency multiplier of 2. Decade – A frequency multiplier of 10. Frequency Octave Decade 100 Hz 200 Hz 1 k. Hz 5 k. Hz fx 2 fx 10 fx

Bode Plot n Bode plot – A frequency response curve that assumes Ap = 0 d. B until the cutoff frequency is reached.

BJT Amplifiers: Base Circuit Lower Cutoff Frequency

BJT Amplifiers: Collector Circuit Lower Cutoff Frequency

BJT Amplifiers: Emitter Circuit Lower Cutoff Frequency

Gain Roll-Off n Roll-off rate – The rate of gain reduction for a circuit when operated beyond its cutoff frequencies. n n The roll-off rate for a given RC circuit is independent of the values of R and C. The low-frequency roll-off rate for an RC circuit is determined using

Roll-off rates n Voltage gain roll-off rates (for RC circuits) are 20 d. B per decade and 6 d. B per octave.

Roll-Off Rates Are Additive

Amplifier Low-Frequency Response n The Bode plot is ideal. The frequency response curve is a more practical representation.

BJT Internal Capacitances n Each BJT junction has some measurable value of capacitance.

Miller’s Theorem n Miller’s theorem – A theorem which states that a feedback capacitor can be represented as separate input and output capacitances.

BJT Amplifier High-Frequency Circuit

High-Frequency Roll-Off n n The high-frequency roll-off rate for a given RC circuit is independent of the values of R and C. The high-frequency roll-off rate for an RC circuit is determined using

FET Amplifier Equivalent Circuit

FET Amplifier Low-Frequency Response

FET Amplifier High-Frequency Response

JFET Capacitance Specifications n n n Ciss – Input capacitance Crss – Reverse transfer capacitance Coss – Output capacitance

Multistage Amplifier Cutoff Frequencies and Bandwidth n When amplifiers having equal cutoff frequencies are cascaded, the cutoff frequencies and bandwidth of the multistage circuit are found using