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Operational Amplifier Gain Stability, Part 1: General System Analysis

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Operational Amplifier Gain Stability, Part 1: General System Analysis Amplifiers: Op Amps Texas Instruments Incorporated Operational amplifier gain stability, Part 1: General system analysis By Miroslav Oljaca, Senior Applications Engineer, and Henry Surtihadi, Analog Design Engineer Introduction In a sinusoidal steady-state analysis, the transfer function The goal of this three-part series of articles is to provide a can be represented as more in-depth understanding of gain error and how it can H( jω= ) H( j ω× ) ej(ϕω ) , (1) be influenced by the actual parameters of an operational amplifier (op amp) in a typical closed-loop configuration. where H( jω ) is the magnitude of the transfer function, This first article explores general feedback control system and ϕ is the phase. Both are functions of frequency. analysis and synthesis as they apply to first-order transfer One way to describe how the magnitude and phase of a functions. This analysis technique is then used to calculate transfer function vary over frequency is to plot them the transfer functions of both noninverting and inverting graphically. Together, the magnitude and phase plots of the op amp circuits. The second article will focus on DC gain transfer function are known as a Bode plot. The magnitude error, which is primarily caused by the finite open-loop part of a Bode diagram plots the expression given by gain of the op amp as well as its temperature dependency. Equation 2 on a linear scale: The third and final article will discuss one of the most H( jω= ) 20 log H( j ω ) (2) dB 10 common mistakes in calculating AC closed-loop gain errors. Often, during circuit analysis, system designers The phase part of a Bode diagram plots the expression have the tendency to use DC-gain calculation methods for given by Equation 3, also on a linear scale: AC-domain analysis, which provides worse results than the ϕ=∠H( j ω ) (3) real performance of the circuit. With these three articles, Both the magnitude and the phase plots are plotted the system designer will have the simple tools required to against a logarithmic frequency axis. determine the overall closed-loop gain error for any specific The benefit of plotting the logarithmic value of the op amp by using its data-sheet parameters. magnitude instead of the linear magnitude of the transfer Steady-state sinusoidal analysis and Bode plots function is the ability to use asymptotic lines to approxi- mate the transfer function. These asymptotic lines can be Before the main topic of this article is discussed, it is drawn quickly without having to use Equation 2 to calcu- appropriate to briefly review the concepts of sinusoidal- late the exact magnitude and can still represent the mag- frequency analysis and Bode plots. These two concepts nitude of the transfer function with reasonable accuracy. will be used repeatedly throughout this series of articles. As an example, consider a first-order (single-pole) It is often useful to characterize a circuit by measuring transfer function, its response to sinusoidal input signals. Fourier analysis can 1 be used to reconstruct any periodic signal by summing sinu- H( jω= ) , (4) 1jω soidal signals with various frequencies. Thus, the circuit + ω designers can gather useful information about a circuit’s 0 response to various input signals by characterizing its where ω0 is the angular cutoff frequency of the system. response to sinusoidal excitations over a wide frequency The magnitude, in decibels, of the transfer function from range. Equation 4 can be described by Equation 5: When a linear circuit is driven by a sinusoidal input 1 H(jω= ) 20 log (5) signal of a specific frequency, the output signal is also a dB 1j+ ω sinusoidal signal of the same frequency. The complex ω0 representation of a sinusoidal waveform can be used to represent the input signal as The transfer function, H(jω), is a complex function of the angular frequency, ω. To calculate the magnitude, both real j(ω t +ϕ1 ) v11 (t)=× V e , and imaginary portions of the function need to be used: and the output signal as 1 H( jω= ) 20 log (6) j(ω t +ϕ2 ) dB v22 (t)=× V e . 2 1+ ω V1 and V2 are the amplitudes of the input and output sig- 2 ω0 nals, respectively; and ϕ1 and ϕ2 are the phase of the input and output signals, respectively. The ratio of the output Equation 6 shows that at a frequency much lower than signals to the input signals is the transfer function, H(jω). ω0, the magnitude is near 1 V/V or 0 dB. At frequency 20 High-Performance Analog Products www.ti.com/aaj 1Q 2010 Analog Applications Journal Texas Instruments Incorporated Amplifiers: Op Amps Figure 1. Bode plot of single-pole transfer function 5 15 0 0 –5.7 –3 –20 dB/dec –5 –15 Asymptotes Amplitude –10 –30 –15 –45 Amplitude (dB) –20 –60 Phase (degrees) Phase –25 –75 –84.3 –30 –90 0.1 110 0 (rad/sec) – ω = ω0, the magnitude drops to 1/√2 = 0.707, or roughly Deriving noninverting and inverting –3 dB. Above this frequency, the magnitude rolls off at a transfer functions rate of –20 dB/decade. For simplicity, all the feedback networks in this article are Both the real and imaginary parts of the transfer func- shown as resistive networks. However, the analysis shown tion can be used to calculate the phase response as here will also be valid when these resistors are replaced ω with complex feedback networks. ϕ( ω ) =− tan−1 . (7) ω0 Figure 2 depicts a typical noninverting op amp configu- ration. The closed-loop gain of the amplifier is set by two Similarly, when the frequency is much lower than ω , the 0 resistors: the feedback resistor, R , and the input resistor, phase is 0°. At the frequency ω = ω , the phase is –45°. F 0 R . The amount of output voltage, V , fed back to the Finally, when the frequency is much higher than ω , the I OUT 0 feedback point is represented by the parameter β. The phase levels off at –90°. feedback point is the inverting input of the op amp. As Figure 1 shows the Bode plot of the first-order transfer stated, the β network is a simple resistive feedback net- function just described. Notice the use of the two asymp- work. From Figure 2, β is defined as totic lines to simplify the magnitude plot of the transfer VR function. At the intersection of the two asymptotic lines, β=FB = I . (8) V RR+ the simplified magnitude curve is off from the actual mag- OUT I F nitude by about 3 dB. At frequencies much lower or much higher than ω0, the error is negligible. Figure 2. Typical noninverting op amp circuit with feedback Feedback Network Network RF VOUT VFB RI - RF VOUT VFB + RI VIN 21 Analog Applications Journal 1Q 2010 www.ti.com/aaj High-Performance Analog Products Amplifiers: Op Amps Texas Instruments Incorporated Figure 3 shows the control-loop model of the circuit in Figure 2. The parameter A is the open-loop gain of the Figure 3. Control-loop model of noninverting OL op amp circuit op amp and is always specified in any op amp data sheet. The control-loop model from Figure 3 can be used to express the closed-loop gain as VFB VA A.==OUT OL (9) CL V 1A+β× IN OL Assuming that this model is of a first-order system, the - open-loop gain of an op amp as a function of angular V + ERR A frequency can be described as VIN OL VOUT AOL _ DC AOL (jω= ) . (10) 1j+ ω ω0 The parameter AOL_DC in Equation 10 is the open-loop ACL(jω) is a complex function of the angular frequency, ω. gain of the op amp at a low frequency or at the DC level. Also recall that to calculate the magnitude, both real and The dominant pole of the op amp is given by the angular imaginary portions of the function need to be used in the frequency, ω0, or equivalently by f0 = ω0/2π. same way that Equation 6 was obtained: The Bode plot of the open-loop gain expression from A Equation 10 is presented in Figure 4. Asymptotic curves OL _ DC 1A+β× are used in this figure to create a simplified version of the A ( jω= ) 20 log OL _ DC (12) CL dB actual open-loop response. Now it is possible to express ω2 1 1+× the closed-loop gain from Equation 9 in the frequency 22 ω0 (1 +β× AOL _ DC ) domain by replacing the parameter AOL with Equation 10. After a few algebraic steps, the closed-loop transfer func- If the angular frequency, ω, is replaced with 2πf, the tion can be written as closed-loop transfer function from Equation 12 can be AOL _ DC rewritten as 1A+β× A (jω= ) OL _ DC . (11) CL ω 1 A 1j+× OL _ DC ω0 1A +β× OL _ DC 1A+β× A ( jf )= 20 log OL _ DC . CL dB (13) f12 1+× 22 f0 (1+β× AOL _ DC ) Figure 4. Open-loop gain vs. frequency of typical op amp 140 AOL_DC f0 120 100 80 60 40 Voltage Gain (dB) Voltage 20 0 –20 10 100 1 k 10 k 100 k1 M 10 M 100 M Frequency (Hz) 22 High-Performance Analog Products www.ti.com/aaj 1Q 2010 Analog Applications Journal Texas Instruments Incorporated Amplifiers: Op Amps Figure 5 depicts a typical inverting op Figure 5.
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