Amplifier Frequency Response Bandwidth: Understanding Bandwidth and Cutoff Frequencies

1. Chapter 14 Amplifier Frequency Response

2. Bandwidth • 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.

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

4. Geometric Center Frequency • Geometric center frequency (f0) – The frequency that equals the geometric average of fC1 and fC2. • Power gain is maximum when an amplifier is operated at its geometric center frequency.

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

6. dB Power Gain Example: Convert Ap = 1200 to dB form.

7. dB Gains Are Additive

8. Frequency Intervals • Octave – A frequency multiplier of 2. • Decade – A frequency multiplier of 10.

9. Bode Plot • Bode plot – A frequency response curve that assumes Ap = 0 dB until the cutoff frequency is reached.

10. BJT Amplifiers: Base Circuit Lower Cutoff Frequency

11. BJT Amplifiers: Collector Circuit Lower Cutoff Frequency

12. BJT Amplifiers: Emitter Circuit Lower Cutoff Frequency

13. Gain Roll-Off • Roll-off rate – The rate of gain reduction for a circuit when operated beyond its cutoff frequencies. • 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

14. Roll-off rates • Voltage gain roll-off rates (for RC circuits) are 20 dB per decade and 6 dB per octave.

15. Roll-Off Rates Are Additive

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

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

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

19. BJT Amplifier High-Frequency Circuit

20. High-Frequency Roll-Off • 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

21. FET Amplifier Equivalent Circuit

22. FET Amplifier Low-Frequency Response

23. FET Amplifier High-Frequency Response

24. JFET Capacitance Specifications • Ciss – Input capacitance • Crss – Reverse transfer capacitance • Coss– Output capacitance

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