1. 2 Microelettronica – Circuiti integrati analogici 2/ed Richard C. Jaeger, Travis N. Blalock Copyright © 2005 – The McGraw-Hill Companies srl Chapter10 Operational Amplifier Applications Microelectronic Circuit Design Richard C. Jaeger Travis N. Blalock

2. 2 Microelettronica – Circuiti integrati analogici 2/ed Richard C. Jaeger, Travis N. Blalock Copyright © 2005 – The McGraw-Hill Companies srl Continue study of methods to determine transfer functions of circuits containing op amps. Introduction to active filters and switched capacitor circuits Explore digital-to-analog converter specifications and basic circuit implementations. Study analog-to-digital converter specifications and implementations. Explore applications of op amps in nonlinear circuits, such as precision rectifiers. Provide examples of multivibrator circuits employing positive feedback. Demonstrate use of ac analysis capability of SPICE. Chapter Goals

3. 2 Microelettronica – Circuiti integrati analogici 2/ed Richard C. Jaeger, Travis N. Blalock Copyright © 2005 – The McGraw-Hill Companies srl Op amp is voltage follower with unity gain over a wide range of frequencies. Uses positive feedback through C 1 at frequencies above dc to realize complex poles without inductors. Feedback network provides dc path for amplifier’s input bias currents. The transfer function is: Active Filters: Low-pass (Transfer Function) In standard form, Often, circuits are designed with C 1 = C 2 = C.

4. 2 Microelettronica – Circuiti integrati analogici 2/ed Richard C. Jaeger, Travis N. Blalock Copyright © 2005 – The McGraw-Hill Companies srl Active Filters: Low-pass (Frequency Response) For Q=0.71,magnitude response is maximally flat (Butterworth Filter: Maximum bandwidth without peaking) For Q>0.71, response shows undesired peaking. For Q<0.71: Filter’s bandwidth capability is wasted. Sensitivity, S represents fractional change in parameter, P due to a given fractional change in value of Z. Sensitivity of with respect to R and C is: At  <<  o, filter has unity gain. At  >>  o,response exhibits two- pole roll-off at 40dB/decade. At  =  o, gain of filter =Q.

5. 2 Microelettronica – Circuiti integrati analogici 2/ed Richard C. Jaeger, Travis N. Blalock Copyright © 2005 – The McGraw-Hill Companies srl Active Filters: Low-pass (Example) Problem: Design second-order low-pass filter with maximally flat response. Given data: f H = 5 kHZ Analysis:C 1 = 2C 2 = 2C and R 1 = R 2 = R. 1/  o C is the reactance of C at  o, R is 30% smaller than this value. Thus impedance level of filter is set by C. If impedance level is too low, op amp will not be able to supply current required to drive feedback network. At 5 kHz, for a 0.01  F capacitor, Final values: = R 1 = R 2 = 2.26k  C 1 = 0.02  F, C 2 = 0.01  F

6. 2 Microelettronica – Circuiti integrati analogici 2/ed Richard C. Jaeger, Travis N. Blalock Copyright © 2005 – The McGraw-Hill Companies srl Active Filters: High-pass with Gain (Transfer Function) Voltage follower in low-pass filter replaced by non-inverting amplifier with gain K, which gives an added degree of freedom in design. dc paths for both op amp input bias currents through R 2 and feedback resistors. The transfer function is: For R 1 = R 2 = R and C 1 = C 2 = C, For K=3, Q is infinite, poles are on j  axis causing sinusoidal oscillations. K>3 causes instability due to right-half plane poles.

7. 2 Microelettronica – Circuiti integrati analogici 2/ed Richard C. Jaeger, Travis N. Blalock Copyright © 2005 – The McGraw-Hill Companies srl Active Filters: High-pass with Gain (Frequency Response) For Q=0.71,magnitude response is maximally flat (Butterworth Filter response). Amplifier gain is constant at  >  o, the lower cutoff frequency of the filter.

8. 2 Microelettronica – Circuiti integrati analogici 2/ed Richard C. Jaeger, Travis N. Blalock Copyright © 2005 – The McGraw-Hill Companies srl Active Filters: Band-pass (Transfer Function) For C 1 = C 2 = C, Uses inverting op amp and its full loop gain (ideally infinite).

9. 2 Microelettronica – Circuiti integrati analogici 2/ed Richard C. Jaeger, Travis N. Blalock Copyright © 2005 – The McGraw-Hill Companies srl Active Filters: Band-pass (Frequency Response) Response peaks at  o and gain at center frequency is 2Q 2. At  >  o, filter response corresponds to single-pole high-pass or low-pass filter changing at a rate of 20dB/decade.

10. 2 Microelettronica – Circuiti integrati analogici 2/ed Richard C. Jaeger, Travis N. Blalock Copyright © 2005 – The McGraw-Hill Companies srl Active Filters: Tow-Thomas Biquad General biquadratic transfer function to represent low-pass, high-pass, band-pass, all-pass and notch filters: In Tow-Thomas biquad, first op amp is a multi-input integrator and third op amp is simply an inverter. Thus, center frequency, Q and gain can each be adjusted independently. Continua…