ITT Technical Institute ET245 Devices II Unit 5 Chapter

ITT Technical Institute ET245 Devices II Unit 5 Chapter 7.1 7.3

Unit 5 Agenda Lecture: Chapter 7, Sections 7.1 7.3 Lab 3, Linear Op amp Circuits continued from last week Assignment: Complete Problems (pg 413): #14, 8, 10, & 12 (Question 12: assume Aol = 89dB = 28,184) Read Chapter 8, Sections 8.1 8.3 Prepare for Quiz #5 on Unit 5

Class Objectives: Unit 5 Objective Learn Op-Amp Basic Concepts of Gain Learn OP-Amp Basic Concepts of Bandwidth

Chapter 7 Section 7.1 Op-Amp Responses - Basic Concepts Open-Loop Gain The open-loop gain (A ol ) of an op-amp is the internal voltage gain of the device and represents the ratio of output voltage to input voltage. Notice no external connections. Data sheets often refer to the open-loop gain as large-signal voltage gain.

Op-Amp Responses - Basic Concepts Closed-Loop Gain The closed-loop gain (A cl ) is the voltage gain of an op-amp with external feedback. The closed-loop gain is determined by the external components. Closed-loop gain can be precisely controlled by external component values.

Op-Amp Responses - Basic Concepts Gain is Frequency Dependant In chapter 6, all of the gain expressions applied to the midrange gain and were considered independent of the frequency. The midrange open-loop gain of an op-amp extends from zero frequency (dc) up to a critical frequency at which the gain is 3 db less than the midrange value.

Op-Amp Responses - Basic Concepts Gain is Frequency Dependant (continued) An open-loop response curve (Bode plot) for a certain op-amp is shown. Most of the op-amp data sheets show this type of curve or specify the midrange open-loop gain. The critical (cutoff) frequency is 10Hz.

Op-Amp Responses - Basic Concepts 3 db Open-Loop Bandwidth The bandwidth of an ac amplifier is the frequency range between the points where the gain is 3 db less than the midrange gain. Therefore, bandwidth is equal to the upper critical frequency minus the lower critical frequency. BW = f cu f cl Since the lower critical frequency is zero (dc), the bandwidth is simply the upper critical frequency. BW = f cu ( referred to as f c )

Op-Amp Responses - Basic Concepts Unity-Gain Bandwidth Note that the gain steadily decreases to a point where it is equal to one (0 db). The value of frequency at which this unity gain occurs is the unity-gain bandwidth.

Op-Amp Responses - Basic Concepts Gain-Versus-Frequency Analysis The RC lag (low-pass) networks within an op-amp are responsible for the roll-off in gain as frequency increases. Attenuation is expressed as: V out X C 1 = = V in R 2 + X C 2 1 + R 2 / X C FIGURE 7-3 RC lag network. = 1 1 + f 2 / f c 2

Op-Amp Responses - Basic Concepts Gain-Versus-Frequency Analysis (continued) If an op-amp is represented by a voltage gain element with a gain of A ol(mid) and a single RC lag network, then the total open-loop gain of the op-amp is the product of the midrange open-loop gain A ol(mid) and the attenuation of the RC network. A ol(mid) A ol = 1 + f 2 / f c 2 FIGURE 7-4 Op-amp represented by gain element and internal RC network.

Op-Amp Impedances EXAMPLE 7.1: Determine A ol for the following values of f. Assume f c(ol) = 100 Hz, and A ol(mid) = 100,000. a) f = 0 Hz b) f = 10 Hz c) f = 100 Hz d) f = 1000 Hz Solution: pg 385

Op-Amp Responses - Basic Concepts Phase Shift As you know, an RC network causes a propagation delay from the input to output, thus creating a phase shift between the input signal and the output signal. = -tan -1 ( f / f c )

Op-Amp Impedances EXAMPLE 7.2: Calculate the phase shift for an RC lag circuit for each of the following frequencies, and than plot the curve of phase shift versus frequency. Assume f c = 100 Hz. Solution: pg 386-387

Chapter 7 Section 7.2 Op-Amp Open-Loop Response Frequency Response In section 7.1, an op-amp was assumed to have a constant roll-off of -20 db / decade above its critical frequency. Op-amps with this characteristic are called compensated op-amps. Some op-amps are more complex. The frequency response may be determined by several internal stages, each with its own critical frequency. An opamp with more than one critical frequency is called an uncompensated op-amp.

Op-Amp Open-Loop Response Frequency Response (continued) Uncompensated op-amps require careful attention to the feedback network to avoid oscillation. Since the roll-off rates are additive, the total roll-off rate increases by -20 db/decade as each critical frequency is reached.

Op-Amp Open-Loop Response Phase Response In a multistage amplifier, each stage contributes to the total phase lag. As you have seen, an RC lag circuit can produce up to a -90 phase shift. A 3-stage op-amp can have up to a -270 phase shift. The phase lag of each stage is less than -45 when the frequency is below the critical frequency, equal to -45 when at the critical frequency, and greater than -45 when the frequency is above the critical frequency.

Op-Amp Open-Loop Response Phase Response (continued) The phase shift lags of the stages of an op-amp are added to produce a total phase lag, according to the following formula for three stages: tot = -tan -1 ( f / f c1 ) -tan -1 ( f / f c2 ) -tan -1 ( f / f c3 )

Op-Amp Impedances EXAMPLE 7.3: A certain op-amp has three internal amplifier stages with the following gains and critical frequencies. Stage 1: A v1 = 40 db, f c1 = 2000 Hz Stage 2: A v2 = 32 db, f c2 = 40 khz Stage 3: A v3 = 20 db, f c3 = 150 khz Determine the open-loop midrange db gain and the total phase lag when f = f c1. Solution: pg 389

Chapter 7 Section 7.3 Op-Amp Closed-Loop Response Effect of Negative Feedback on Bandwidth The closed-lop critical frequency of an op-amp is: f c(cl) = f c(ol) (1 + BA ol(mid) ) where B is the feedback attenuation BW cl = BW ol (1 + BA ol(mid) )

Op-Amp Impedances EXAMPLE 7.4: A certain op-amp has an open-loop midrange gain of 150,000 and an open-loop 3 db bandwidth of 200 Hz. The attenuation of the feedback loop is 0.002. What is the closed-loop bandwidth? Solution: pg 390

Op-Amp Closed-Loop Response Effect of Negative Feedback on Bandwidth When the open-loop gain of an op-amp is reduced by negative feedback, the bandwidth is increased.

Op-Amp Impedances EXAMPLE 7.5: Determine the bandwidth of each of the amplifiers in figure. Both op-amps have an open-loop gain of 100 db and a unity-gain bandwidth of 3 MHz.? Solution: pg 392

Op-Amp Closed-Loop Response Gain-Bandwidth Product An increase in closed-loop gain causes a decrease in the bandwidth and vice versa, such that the product of gain and bandwidth is a constant (as long as the roll-off rate is a fixed -20 db/decade). The gain-bandwidth product is always equal to the frequency at which the op-amp s open-loop gain is unity (unity-gain bandwidth). A cl f c(cl) = unity-gain bandwidth

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