Operational Amplifier Circuits

Operational Amplifier Circuits Review: Ideal Op-amp in an open loop configuration Ip + Vp + Vi _ Vn Ro + Ri Vo AVi _ In An ideal op-amp is characterized with infinite open–loop gain A→∞ The other relevant conditions for an ideal op-amp are: 1. Ip = In = 0 2. Ri = ∞ 3. Ro = 0 Ideal op-amp in a negative feedback configuration When an op-amp is arranged with a negative feedback the ideal rules are: 1. Ip = In = 0 : input current constraint 2. Vn = Vp : input voltage constraint These rules are related to the requirement/assumption for large open-loop gain A → ∞ , and they form the basis for op-amp circuit analysis. The voltage Vn tracks the voltage Vp and the “control” of Vn is accomplished via the feedback network. Chaniotakis and Cory. 6.071 Spring 2006 Page 1 Operational Amplifier Circuits as Computational Devices So far we have explored the use of op amps to multiply a signal by a constant. For the inverting amplifier the multiplication constant is the gain − RR12 and for the non inverting amplifier the multiplication constant is the gain 1+ RR12 . Op amps may also perform other mathematical operations ranging from addition and subtraction to integration, differentiation and exponentiation.1 We will next explore these fundamental “operational” circuits. Summing Amplifier A basic summing amplifier circuit with three input signals is shown on Figure 1. Vin1 Vin2 Vin3 R1 I1 R2 I2 R3 I3 IF RF N1 Vout Figure 1. Summing amplifier Current conservation at node N1 gives I1 + I 2 + I 3 = I F (1.1) By relating the currents I1, I2 and I3 to their corresponding voltage and resistance by Ohm’s law and noting that the voltage at node N1 is zero (ideal op-amp rule) Equation (1.1) becomes Vin1 Vin 2 Vin 3 V + + = − out R1 R 2 R3 RF (1.2) 1 The term operational amplifier was first used by John Ragazzini et. al in a paper published in 1947. The relevant historical quotation from the paper is: “As an amplifier so connected can perform the mathematical operations of arithmetic and calculus on the voltages applied to its inputs, it is hereafter termed an ‘operational amplifier’.” John Ragazzini, Robert Randall and Frederick Russell, “ Analysis of Problems in Dynamics by Electronics Circuits,” Proceedings of IRE, Vol. 35, May 1947 Chaniotakis and Cory. 6.071 Spring 2006 Page 2 And so Vout is RF RF ⎛ RF ⎞ Vout = − ⎜ Vin1 + Vin 2 + Vin 3 ⎟ R2 R2 ⎝ R1 ⎠ (1.3) The output voltage Vout is a sum of the input voltages with weighting factors given by the values of the resistors. If the input resistors are equal R1=R2=R3=R, Equation (1.3) becomes Vout = − RF (Vin1 + Vin 2 + Vin3 ) R (1.4) The output voltage is thus the sum of the input voltages with a multiplication constant RF given by . The value of the multiplication constant may be varied over a wide range R and for the special case when RF = R the output voltage is the sum of the inputs Vout = − (Vin1 + Vin 2 + Vin 3 ) (1.5) The input resistance seen by each source connected to the summing amplifier is the corresponding series resistance connected to the source. Therefore, the sources do not interact with each other. Chaniotakis and Cory. 6.071 Spring 2006 Page 3 Difference Amplifier This fundamental op amp circuit, shown on Figure 2, amplifies the difference between the input signals. The subtracting feature is evident from the circuit configuration which shows that one input signal is applied to the inverting terminal and the other to the noninverting terminal. Vin1 Vin2 R1 I1 IF R2 N1 R3 Vout I2 R4 Figure 2. Difference Amplifier Before we proceed with the analysis of the difference amplifier let’s think about the overall behavior of the circuit. Our goal is to obtain the difference of the two input signals (Vin 2 - Vin1 ) . Our system is linear and so we may apply superposition in order to find the resulting output. We are almost there once we notice that the contribution of the signal Vin2 to the output is ⎛ R4 ⎞ ⎛ R2 ⎞ Vout 2 = Vin 2 ⎜ ⎟ ⎜1 + ⎟ R1 ⎠ ⎝ R3 + R 4 ⎠ ⎝ (1.6) and the contribution of signal Vin1 is ⎛ R2 ⎞ Vout1 = -Vin1 ⎜ ⎟ ⎝ R1 ⎠ (1.7) R2 ⎛ R4 ⎞ ⎛ R2 ⎞ Vout = Vout 2 - Vout1 = Vin 2 ⎜ ⎟ ⎜1 + ⎟ - Vin1 R1 ⎠ R1 ⎝ R3 + R 4 ⎠ ⎝ (1.8) And the output voltage is Chaniotakis and Cory. 6.071 Spring 2006 Page 4 Note that in order to have a subtracting circuit which gives Vout=0 for equal inputs, the weight of each signal must be the same. Therefore ⎛ R4 ⎞ ⎛ R2 ⎞ R2 ⎜ ⎟ ⎜1 + ⎟= R1 ⎠ R1 ⎝ R3 + R 4 ⎠ ⎝ (1.9) R4 R2 = R3 R1 (1.10) R2 (Vin 2 - Vin1 ) R1 (1.11) which holds only if The output voltage is now Vout = which is a difference amplifier with a differential gain of R2/R1 and with zero gain for the common mode signal. It is often practical to select resistors such as R4=R2 and R3=R1. The fundamental problem of this circuit is that the input resistance seen by the two sources is not balanced. The input resistance between the input terminals A and B, the V differential input resistance, Rid (see Figure 3) is Rid ≡ in I A + V in B I R1 V - R3 V + R2 Vout R4 Figure 3. Differential amplifier Since V+ = V- , Vin = R1 I + R3 I and thus Rid = 2 R1 . The desire to have large input resistance for the differential amplifier is the main drawback for this circuit. This problem is addressed by the instrumentation amplifier discussed next. Instrumentation Amplifier Figure 4 shows our modified differential amplifier called the instrumentation amplifier (IA). Op amps U1 and U2 act as voltage followers for the signals Vin1 and Vin2 which see the infinite input resistance of op amps U1 and U2. Assuming ideal op amps, the voltage Chaniotakis and Cory. 6.071 Spring 2006 Page 5 at the inverting terminals of op amps U1 and U2 are equal to their corresponding input voltages. The resulting current flowing through resistor R1 is V −V (1.12) I1 = in1 in 2 R1 Since no current flows into the terminals of the op amp, the current flowing through resistor R2 is also given by Equation (1.12). Vin1 V01 U1 V1 R4 R2 R3 R1 U3 R3 Vout R2 R4 V2 V02 U2 Vin2 Figure 4. Instrumentation Amplifier circuit Since our system is linear the voltage at the output of op-amp U1 and op-amp U2 is given by superposition as R2 ⎛ R2 ⎞ V01 = ⎜ 1 + Vin2 ⎟ Vin1 − R1 ⎠ R1 ⎝ (1.13) R2 ⎛ R2 ⎞ V02 = ⎜ 1 + Vin1 ⎟ Vin2 − R1 ⎠ R1 ⎝ (1.14) Next we see that op amp U3 is arranged in the difference amplifier configuration examined in the previous section (see Equation (1.11)). The output of the difference amplifier is Vout = The differential gain, R4 ⎛ 2R2 ⎞ ⎜1 + ⎟ ( Vin2 − Vin1 ) R3 ⎝ R1 ⎠ (1.15) R4 ⎛ 2R2 ⎞ ⎜1 + ⎟ , may be varied by changing only one resistor: R1. R3 ⎝ R1 ⎠ Chaniotakis and Cory. 6.071 Spring 2006 Page 6 Current to voltage converters A variety of transducers produce electrical current in response to an environmental condition. Photodiodes and photomultipliers are such transducers which respond to electromagnetic radiation at various frequencies ranging from the infrared to visible to γrays. A current to voltage converter is an op amp circuit which accepts an input current and gives an output voltage that is proportional to the input current. The basic current to voltage converter is shown on Figure 5. This circuit arrangement is also called the transresistance amplifier. R N1 Vout I in Figure 5. Current to voltage converter Iin represents the current generated by a certain transducer. If we assume that the op amp is ideal, KCL at node N1 gives ⎛V −0 ⎞ I1 + ⎜ out ⎟ = 0 ⇒ Vout = − RI1 ⎝ R ⎠ (1.16) The “gain” of this amplifier is given by R. This gain is also called the sensitivity of the converter. Note that if high sensitivity is required for example 1V/µV then the resistance R should be 1 MΩ. For higher sensitivities unrealistically large resistances are required. A current to voltage converter with high sensitivity may be constructed by employing the T feedback network topology shown on Figure 6. In this case the relationship between Vout and I1 is ⎛ R2 R2 ⎞ Vout = − ⎜ 1 + + ⎟ I1 R1 R ⎠ ⎝ Chaniotakis and Cory. 6.071 Spring 2006 (1.17) Page 7 R N1 R1 R2 Vout I in Figure 6. Current to voltage converter with T network Chaniotakis and Cory. 6.071 Spring 2006 Page 8 Voltage to Current converter A voltage to current (V-I) converter accepts as an input a voltage Vin and gives an output current of a certain value. In general the relationship between the input voltage and the output current is I out = SVin (1.18) Where S is the sensitivity or gain of the V-I converter. Figure 7 shows a voltage to current converter using an op-amp and a transistor. The opamp forces its positive and negative inputs to be equal; hence, the voltage at the negative input of the op-amp is equal to Vin. The current through the load resistor, RL, the transistor and R is consequently equal to Vin/R. We put a transistor at the output of the op-amp since the transistor is a high current gain stage (often a typical op-amp has a fairly small output current limit). Vcc RL V in R Figure 7. Voltage to current converter Chaniotakis and Cory. 6.071 Spring 2006 Page 9 Amplifiers with reactive elements We have seen that op amps can be used with negative feedback to make simple linear signal processors. Examples include amplifiers, buffers, adders, subtractors, and for each of these the DC behavior described the apparent behavior over all frequencies. This of course is a simplification to treat the op amp ideally, as through it does not contain any reactive elements. Providing we keep the operating conditions out of the slew rate limit then this is a reasonable model. Here we wish to extend this picture of op amp operation to include circuits that are designed to be frequency dependent. This will enable the construction of active filters, integrators, differentiators and oscillators. The feedback network of an op-amp circuit may contain, besides the resistors considered so far, other passive elements. Capacitors and inductors as well as solid state devices such as diodes, BJTs and MOSFETs may be part of the feedback network. In the general case the resistors that make up the feedback path may be replaced by generalized elements with impedance Z1 and Z2 as shown on Figure 8 for an inverting amplifier. Z2 Z1 Vout Vin Figure 8. Inverting amplifier with general impedance blocks in the feedback path. For an ideal op-amp, the transfer function relating Vout to Vin is given by Z (ω ) Vout =− 2 Vin Z1 (ω ) (1.19) Now, the gain of the amplifier is a function of signal frequency (ω) and so the analysis is to be performed in the frequency domain. This frequency dependent feedback results in some very powerful and useful building blocks. Chaniotakis and Cory. 6.071 Spring 2006 Page 10 The Integrator: Active Low Pass Filter The fundamental integrator circuit (Figure 9) is constructed by placing a capacitor C, in the feedback loop of an inverting amplifier. C IC R V in V out IR R Figure 9. The integrator circuit Assuming an ideal op-amp, current conservation at the indicated node gives I R = IC (1.20) Vin dV = −C out R dt Rearranging Equation (1.20) and integrating from 0 to t, we obtain ∫ dVout = − ∫ Vin (τ ) dτ RC t ⇒ Vout (t ) = − 1 Vin (τ ) dτ + Vout (0) RC ∫0 (1.21) The output voltage is thus the integral of the input. The voltage Vout (0) is the constant of integration and corresponds to the capacitor voltage at time t = 0. The frequency domain analysis is obtained by expressing the impedance of the feedback components in the complex plane. The transfer function may thus be written as 1 Vout Z j jωC =− C = = Vin ZR R ω RC Chaniotakis and Cory. 6.071 Spring 2006 (1.22) Page 11 The above expression indicates that there is a 90o phase difference between the input and the output signals. This 90o phase shift occurs at all frequencies. The gain of the amplifier V 1 is also a function of frequency. For dc signals with given by the modulus out = Vin ω RC ω=0 the gain is infinite and it falls at a rate of 20dB per decade of frequency change. The infinite gain for dc signals represents a practical problem for the circuit configuration of Figure 27. Since the equivalent circuit of a capacitor for ω=0 is an open circuit, the feedback path is open. This lack of feedback results in a drift (cumulative summing) of the output voltage due to the presence of small dc offset voltages at the input. This problem may be overcome by connecting a resistor, RF, in parallel with the feedback capacitor C as shown on Figure 10. C IC RF R V in V out IR R Figure 10. Active Low Pass filter The feedback path consists of the capacitor C in parallel with the resistor RF. The equivalent impedance of the feedback path is RF R Ζ RF jωC ZF = F C = = 1 1 + jω RF C RF + ΖC R + F jωC The transfer function (1.23) Z (ω ) Vout becomes =− F Vin Z1 (ω ) Z (ω ) Vout R R 1 1 =− F =− F =− F Vin Z1 (ω ) R1 1 + jω RF C R1 1 + jω (1.24) ωΗ Chaniotakis and Cory. 6.071 Spring 2006 Page 12 Where ωH ≡ 1 RF C (1.25) Vout versus frequency. At frequencies much less Vin R than ωH (ω << ωH) the voltage gain becomes equal to F , while at frequencies higher R than ωH (ω >> ωH) the gain decreases at a rate of 20dB per decade. Figure 11 shows the logarithmic plot of Figure 11. Bode plot of active low pass filter with a gain of 5. So we have seen that the integration is achieved by charging the feedback capacitor. For an integrator to be useful it must be allowed to be reset to zero. Since the output is stored in the charge of the feedback capacitor, all we need to do is to short out the capacitor in order to reset the integrator. Integrators are very sensitive to DC drift, small offsets lead to a steady accumulation of charge in the capacitor until the op amp output saturates. We can avoid this by providing another feedback path for DC. The circuit incorporates a shorting path across the capacitor as shown on Figure 12. Chaniotakis and Cory. 6.071 Spring 2006 Page 13 C IC RF R V in V out IR R Figure 12. Integrator with reset button Chaniotakis and Cory. 6.071 Spring 2006 Page 14 The Differentiator: Active High Pass Filter A differentiator circuit may be obtained by replacing the capacitor with an inductor for Figure 9. In practice this is rarely done since inductors are expensive, bulky and inefficient devices. Figure 13 shows a fundamental differentiator circuit constructed with a capacitor and a resistor. R C V out V in R Figure 13. The differentiator circuit For an ideal op-amp, the current flowing through the capacitor, C current flowing through the resistor, dVin , is equal to the dt Vout , and thus R Vout = − RC dVin dt (1.26) The output is thus proportional to the derivative of the input. As the integrator is sensitive to DC drifts, the differentiator is sensitive to high frequency noise. The differentiator thus is a great way to search for transients, but will add noise. The integrator will decrease noise. Both of these arguments assume the common situation of the noise being at higher frequency than the signal. Chaniotakis and Cory. 6.071 Spring 2006 Page 15 Active Band Reject Filter The integrator and differentiator demonstrate that op amp circuits can be designed to be frequency dependent. This permits the design of active filters, a filter that has gain. We saw before that we could design passive filters based on LC circuits, active filters are no more complicated. Simple selective filters can be made through a frequency dependent impedance in the feedback loop. Consider the band reject circuit shown on Figure 14. C L RF IF R1 V in V out IR R Figure 14. Active band reject filter We can understand how this circuit works without any detailed calculations. All we need to do is look at the feedback loops. There are two paths in the feedback loop: a frequency independent path with resistance RF, and a frequency dependent path with impedance given by 1 ⎞ ⎛ Zω = j ⎜ ω L − ωC ⎟⎠ ⎝ (1.27) Let’s look at the behavior of the circuit as a function of frequency. For DC signals ( ω = 0 ) the capacitor acts as an open circuit and the equivalent circuit is shown on Figure 15. Chaniotakis and Cory. 6.071 Spring 2006 Page 16 RF IF R1 V in V out IR R Figure 15 Similarly at high frequencies ( ω → ∞ ) the inductor acts as an open circuit and the equivalent circuit is the same as the one shown on Figure 15. Therefore the voltage transfer characteristics at DC and at high frequency are the same with a gain given by G= Vout R =− F Vin R1 (1.28) The other frequency of interest if the resonance frequency, which occurs when Zω as given by Equation (1.27) is equal to zero. The resonance frequency is 1 LC and the circuit reduces to the one shown on Figure 16. ωo = (1.29) IF R1 V in V out IR R Figure 16 which gives Vout=0 at ω = ωo . Chaniotakis and Cory. 6.071 Spring 2006 Page 17 So this is a filter that passes and amplifies every frequency except the resonance frequency. For the full analysis of this active filter we may write down the complete expression for the impedance of the feedback loop which is 1 ⎞ ⎛ j ⎜ωL − R j (ω 2 LC − 1) RF ωC ⎟⎠ F ⎝ Z F = Zω // RF = = 1 ⎞ ω RF C + j (ω 2 LC − 1) ⎛ RF + j ⎜ ω L − ωC ⎟⎠ ⎝ (1.30) And thus the general transfer function of the filter is j (ω 2 LC − 1) RF Vout ZF =− = Vin R1 R1 ⎡ω RF C + j (ω 2 LC − 1) ⎤ ⎣ ⎦ Chaniotakis and Cory. 6.071 Spring 2006 (1.31) Page 18 Diodes and transistors in op-amp circuits. Diodes and transistors may also be used in op-amp circuits. The nonlinear behavior of these devices result in very interesting and useful non-linear op-amp circuits. Logarithmic Amplifier If we are interested in processing a signal that has a very wide dynamic range we take advantage of the exponential i-v characteristics of the diode and design an amplifier whose output is proportional to the logarithm of the input. In practice we may have the voltage signal that corresponds to a certain chemical activity such as the activity, or concentration, of hydrogen ions in a solution which represents the pH of the solution. In this case the voltage is exponentially related to the concentration (pH). Vi = Vo k ln( pH ) (1.32) If we use this signal as the input to an inverting amplifier we may linearize the signal by using a diode in the feedback path of the amplifier. Recall the i-v relationship for a diode I = Io ⎡⎣ eqV / kT − 1⎤⎦  IoeqV / kT (1.33) Consider the circuit shown on Figure 17. I R1 IR Vi Vn Vo Vp R Figure 17. Logarithmic Amplifier KCL at the indicated node gives Chaniotakis and Cory. 6.071 Spring 2006 Page 19 Vi − Vn = Ioe q (Vn −Vo ) / kT R1 (1.34) Vi = Ioe q ( −Vo ) / kT R1 (1.35) And since Vn = Vp = 0 we obtain And solving for Vo gives the desired relationship. Vo = − kT Vi ln q Io R1 Vo = − kT kT ln(Vi ) + ln( IoR1) q q N  a (1.36) b Vo = − a ln(Vi ) + b Similarly and antilogarithmic amplifier may be constructed by placing the diode in series with the signal source as shown on Figure 18. R2 IF Vi Vn I Vo Vp R Figure 18. Antilogarithmic amplifier Here you may show that Vo = − IoR 2e qVi / kT Chaniotakis and Cory. 6.071 Spring 2006 (1.37) Page 20 Superdiode. Precision half wave Rectifier The diode rectifier circuit and its associated voltage transfer characteristic curve are shown on Figure 19(a) and (b). Vout D Vin + Vd R11 Vout Vd Vin (b) (a) Figure 19. Diode rectifier circuit (a) and voltage transfer curve (b) The offset voltage Vd is about 0.7 Volts and this offset value is unacceptable in many practical applications. The operational amplifier and the diode in the circuit of Figure 20 form an ideal diode, a superdiode, and thus they eliminate the offset voltage Vd from the voltage transfer curve forming an ideal half wave rectifier. Superdiode Vout V- I1 V1 V+ Vin D Id + Vd - I2 R Vout Vin Figure 20. Precision half wave rectifier circuit and its voltage transfer curve. Let’s analyze the circuit by considering the two cases of interest: Vin>0 and Vin<0. For Vin<0 the current I2 and id will be less than zero (point in a opposite direction to the one indicated). However, negative current can not go through the diode and thus the diode is reverse biased and the feedback loop is broken. Therefore the current I2 is zero and so the output voltage is also zero, Vout=0. Since the feedback loop is open the voltage V1 at the output of the op-amp will saturate at the negative supply voltage. Chaniotakis and Cory. 6.071 Spring 2006 Page 21 For Vin>0, Vout=Vin and the current I2=Id and the diode is forward biased. The feedback loop is closed through the diode. Note that there is still a voltage drop Vd across the diode and so the op-amp output voltage V1 is adjusted so that V1=Vd+Vin. Chaniotakis and Cory. 6.071 Spring 2006 Page 22 Problems P1. Resistors R1 and R2 of the circuit on Figure P1 represent two strain gages placed across each other on a beam in order to measure the tensile and compressive stains. R1 and R2 vary symmetrically by a factor δ as R1=R(1-δ) and R2=R(1+δ). Vb R1 + _ R Vs R2 R Figure P1. Design an amplifier so that the output varies from -10V to +10V as the parameter δ varies from -0.01 to +0.01. The bias voltage Vb=+10V and R=10kΩ. P2. The resistors of the amplifier circuit of Figure P2 have a tolerance of ±δ%. 1. Assume that Vin is known precisely and calculate the deviation in the output voltage Vout. 2. For R1=15kΩ±5% and R2=200kΩ±5% and Vin=100mV±1% calculate the output voltage. R2 R1 V out V in Figure P2. Chaniotakis and Cory. 6.071 Spring 2006 Page 23 P3. Calculate the output voltage Vout for the following circuits. V in V out R 3R 2R R2 V in R2 V out 2R 3R R P4 For the circuit on Figure P4 determine the value of resistor Rx so that the output voltage is zero. Rx R2 V out V in R3 R4 Chaniotakis and Cory. 6.071 Spring 2006 Page 24 P5 The circuit on Figure P5 is a current source. 1. Show that the amount of current delivered to the load is controlled by resistor R3. 2. Calculate the resistance seen by the Load. R2 R1 V out V in R4 R3 Load Figure P5 P6. For the circuit on Figure P6 calculate the currents i1, i2, i3, i4 and the voltages v1 and Vout. Refer your answers to the indicated current directions. R2 R1 R4 i2 R3 i1 v1 V in i3 i4 V out Figure P6 Chaniotakis and Cory. 6.071 Spring 2006 Page 25 P7 For the voltage to current converter, briefly (in 1-2 sentences) describe what happens when each of the following faults happen (alone, independently of other faults). a. the Zener diode is shorted b. R5 has a bad soldering joint and is opened c. The load becomes shorted d. The connection between the op-amp output and the base of the transistors becomes opened P8: Op-amp nonidealities: a. What is the effect of a 1uA input bias current on the output voltage of the opamp? b. What is the effect of a 5mV input offset voltage on the output of the opamp? Chaniotakis and Cory. 6.071 Spring 2006 Page 26