Operational Amplifier Circuits: Comparators and Positive Feedback

Operational Amplifier Circuits Comparators and Positive Feedback Comparators: Open Loop Configuration The basic comparator circuit is an op-amp arranged in the open-loop configuration as shown on the circuit of Figure 1. The op-amp is characterized by an open-loop gain A and let’s assume that the output voltage Vo can go all the way to VDD and VEE. The output voltage is given by Vo = A(V+ −V− ) (1.1) Where V+ and V− correspond to the voltages at the non-inverting and the inverting terminals respectively. VDD Vin Vo Vref VEE Figure 1. Basic non-inverting comparator. For the circuit on Figure 1, V+ = Vin and V− = Vref . For Vref = 0 , the voltage transfer characteristic Vo versus Vin is as shown on Figure 2. Vo linear region vδ− saturation saturation VDD Vin vδ+ VEE Figure 2. Voltage transfer characteristic of non-inverting comparator When Vin > vδ + , Vo = VDD and for Vin < vδ − , Vo = VEE . The values vδ + and vδ − is inversely proportional to the open-loop gain A. VDD A VEE = A vδ + = vδ − (1.2) Operation in the linear region is restricted to vδ − < Vin < vδ + . Outside this range the opamp is driven to saturation. For a practical op-amp A=200000 and for VDD=10V and VEE =-10V, vδ + ,− = ±50µV , a very small voltage. Therefore, the amplifier may be driven to saturation very easily. For Vref > 0 , the voltage transfer characteristic Vo versus Vin is as shown on Figure 3. Vo linear region saturation VDD vδ− Vin vδ+ Vref saturation VEE Figure 3. Voltage transfer characteristic of non-inverting comparator with non-zero reference Here the characteristic is shifted to the right by Vref. The range of Vin for operation in the linear region is the same and the saturation voltages are again VDD and VEE. Due to the large value of A, the voltage range [ vδ − , vδ + ] is very small and thus for the remainder of this treatment we will present the behavior of the comparator assuming infinite open-loop gain which corresponds to zero linear region. Therefore, the voltage transfer plot shown on Figure 3 will become as shown on Figure 4. Here we see a transition from one saturation region to the other as the voltage Vin crosses Vref. Vo saturation VDD Vin Vref saturation VEE Figure 4. Ideal comparator The comparator may also be arranged in the inverting configuration as shown on Figure 5 where the input signal is applied to the inverting terminal and the reference voltage is at the non-inverting terminal. VDD Vref Vo Vin VEE Figure 5. Inverting comparator Since Vo = A(Vref −Vin ) the corresponding voltage transfer characteristic is shown on Figure 6. Vo VDD Vin Vref VEE Figure 6 Another arrangement of an non-inverting comparator and its voltage transfer characteristic is shown on Figure 7(a). The output voltage Vo is again Vo = A(V+ −V− ) (1.3) The voltage, V+ , at the non-inverting terminal is found by superposition ant it is V+ = Vin R2 R1 +Vref R1 + R2 R1 + R2 (1.4) Since V− = 0 the comparator will transition when V+ crosses zero. This happens when 0 = Vin (t ) R2 +Vref R1 (1.5) And thus the transition voltage is Vin (t ) = − R1 Vref R2 (1.6) V+ > 0 corresponds to Vin > Vin (t ) and the output goes positive ( VH ). Similarly, V+ < 0 corresponds to Vin < Vin (t ) and the output goes negative ( VL ). The voltage transfer curve is shown on Figure 7(b). Vo Vin Vref VH R1 R2 Rp V+ V- Vin Vo Vin(t) VL (a) (b) Figure 7. Non-inverting comparator circuit The corresponding arrangement for the inverting configuration is shown on Figure 8. Vo Vin Vref R1 R2 Rp VH VV+ Vin Vo Vin (t) VL (a) (b) Figure 8. Inverting comparator and its voltage transfer characteristic. A common application of a comparator circuit is in smoke alarm circuits as shown on Figure 9. Vcc R3 Vcc R1 sealed region provides reference Vref Q1 Vo smoke enters this region Vcc R4 Vin Q2 R2 Vcc Figure 9. Smoke detector schematic The diodes emit light which is detected by the phototransistors Q1 and Q2. The top region is sealed and thus the operating point of transistor Q1 does not change. This operating point is used as the reference for our comparator. When smoke enters the lower region the operating point of phototransistor Q2 changes thereby resulting in a change in the voltage Vin from the base (no smoke) value Vin(ns). The basic voltage characteristic of the comparator is as shown on Figure 10. Vo VH Vin Vin(ns) Vref VL Figure 10 As the intensity of the light at the base of the phototransistor decreases due to smoke entering the region, the base current decreases and the voltage Vin will increase from the base (no smoke) value Vin(ns). When the voltage Vin crosses Vref the output of the comparator switches from VL to VH triggering the alarm. Let’s now consider what happens if the time evolution of the voltage Vin is as shown on Figure 11. Vin Vref t1 t2 t Vo VH t VL Figure 11 Note that as Vin exceeds Vref, the voltage at the output of the comparator switches from VL to VH and the alarm is turned on. Notice that sometime after t1 the voltage Vin begins to decrease and at time t2 is crosses Vref . Now the output of the comparator switches back to VL and the alarm is turned off. This fluctuation in Vin might be noise in the signal or some temporary decreases in the smoke (the effect of a person fanning at the smoke detector). The smoke is still around however and as it is shown on the evolution, the transition voltage will be crossed numerous times turning the alarm on and off each time. This is an undesirable operation and we would like to modify the circuit in such a way that there is some “noise immunity” in the behavior of the circuit. Positive Feedback. Schmitt Trigger For example, if we are able to specify a voltage range within which the state of the output can be at either value then we should be able to build the appropriate level of noise immunity. The conceptual representation of the idea is shown on Figure 12. In this case the alarm turns on at time t1 and turns off at time t2 avoiding all the chatter in between. The circuit that will accomplish this employs positive feedback and it is called Schmitt Trigger. The basic form of the circuit is shown on Figure 13. Positive feedback is achieved by connecting the feedback path to the non-inverting terminal of the op-amp. Vin V1 V2 Immunity band t1 t2 t Vo ON VH t VL OFF OFF Figure 12 Rp Vin V- Vo V+ R1 R2 Figure 13. Inverting Schmitt trigger. In our example, the input signal is applied to the inverting terminal and consequently, the circuit is called Inverting Schmitt Trigger. The voltage V+ is related to the output voltage Vo by the simple voltage divider formed by resistors R1 and R2 . V+ = Vo R1 R1 + R2 (1.7) Note that V+ depends on the output voltage Vo and the in turn Vo depends on the difference between V+ and V− : Vo = A(V+ −V− ) The key to analyzing these positive feedback circuits is to assume an initial condition. The possible values that Vo can take are VH and VL where VH > VL , also assume that VL < 0 . By assuming that the initial state of Vo is VH then, V+ = VH R1 R1 + R2 (1.8) and the output is ⎡ ⎤ R1 Vo = A ⎢VH −Vin ⎥ ⎣ R1 + R2 ⎦ (1.9) R1 the output will remain at VH . R1 + R2 R1 The transition from VH to VL will happen when Vin > VH . This part of the R1 + R2 R1 operation is shown on Figure 14(a) where the voltage VTU ≡ VH . R1 + R2 R1 So when Vin > VTU , Vo = VL and V+ = VL . R1 + R2 Now let’s see what happens when Vin starts decreasing. In order for a transition to happen Therefore, as long as Vin < VH ⎡ ⎤ R1 again the term ⎢VL −Vin ⎥ has to change sign. This will happen when ⎣ R1 + R2 ⎦ R1 R1 as shown on Figure 14(b) where VTL ≡ VL . Vin < VL R1 + R2 R1 + R2 The complete voltage transfer characteristic is obtained by combining Figure 14(a) and (b) and it is shown on Figure 15. The voltage at which the state of the output changes depends on whether the input voltage increases or decreases. When Vin is between VTU and VTL the state of Vo can be either VH or VL - two possible states. For this reason this circuit is also called a Bistable Multivibtator. The voltage transfer characteristic shows that there is Hysteresis Effect. The width of the hysteresis region, VTU −VTL , corresponds to the noise immunity of the trigger circuit. Vo Vo VH VH Vin Vin VTU VTL VL VL (a) (b) Figure 14. Vo VH VTL VTU Vin VL Figure 15. Voltage characteristic of inverting Schmitt trigger Non-Inverting Schmitt Circuit When the input signal is applied to the non-inverting terminals as in the circuit of Figure 16 the resulting circuit is the non-inverting Schmitt trigger. Rp Vin V- Vo V+ R1 R2 Figure 16. Non-inverting Schmitt trigger The voltage V+ at the non-inverting terminal is a combination of the output voltage, Vo , and the input voltage, Vin . V+ = Vin R2 R1 +Vo R1 + R2 R1 + R2 (1.10) Since Vo = A(V+ −V− ) the transitions will occur when V+ crosses zero. 0 = VT R2 +Vo R1 (1.11) Where VT represents the transition voltage. Let’s assume the initial state Vo = VL . Then VT = − R1 VL R2 (1.12) And since VL < 0 , VT is positive and we call it VTU . This corresponding part of the characteristic plot is shown on Figure 17(a). Now the output is at VH and will stay at this state until the voltage V+ crosses zero. This will happen at the transition voltage VT which must satisfy 0 = VT R2 +VH R1 This is the lower transition voltage VTL given by (1.13) R1 (1.14) VH R2 For VH > 0 , VTL < 0 and the corresponding path and transition voltage is shown on Figure 17(b). The complete voltage characteristic is shown on Figure 18. VTL = − Vo Vo VH VH Vin VTU Vin VTL VL VL (a) (b) Figure 17. Vo VH VTL VTU VL Figure 18 Vin Schmitt Trigger with Reference Voltage The circuit on Figure 19(a) is an inverting Schmitt trigger with a reference voltage Vref. The analysis of the circuit proceeds as before. The voltage V+ at the non-inverting terminal is a combination of the output voltage, Vo , and the reference voltage, Vref . V+ = Vref R2 R1 + Vo R1 + R2 R1 + R2 (1.15) Vo Rp VH Vin V- Vref V+ Vo VTU Vin VTL R1 VS R2 VL (b) (a) Figure 19. Inverting Schmitt trigger with reference voltage. (a) circuit, (b) voltage transfer characteristic By comparing Equation (1.15) to Equation (1.8) we see that the location of the transition R2 voltages is simply shifted by VS = Vref . R1 + R2 In this case the transition voltages are: VTU = VS + VH R1 R1 + R2 (1.16) R1 (1.17) R1 + R2 The voltage transfer curve of the inverting Schmitt trigger with reference voltage is shown on Figure 19(b). VTL = VS + VL A non-inverting Schmitt trigger circuit with reference voltage is shown on Figure 20(a) with the corresponding voltage characteristic on Figure 20(b). The transition voltages are given by: VTU = VS −VL R1 R2 (1.18) VTL = VS −VH R1 R2 (1.19) ⎞ ⎟ Vref ⎠ (1.20) Where ⎛ R VS = ⎜1 + 1 ⎝ R2 Therefore, with the appropriate choice of Vref, R1 and R2 we may design a Schmitt trigger with the desired hysteresis characteristics. Vo Rp Vref Vin VH V- Vo V+ VTU Vin VTL R1 VS R2 VL (a) (b) Figure 20. Non-Inverting Schmitt trigger with reference voltage. (a) circuit, (b) voltage transfer characteristic Schmitt Trigger Oscillator: Astable Multivibrator By combining negative and positive feedback and a capacitor for energy storage we can design a circuit that generates a square wave output. The circuit is shown on Figure 21. R2 R1 V+ Vo VRF C Figure 21. Astable Multivibrator Let’s analyze the operation of this circuit for R1 = R2 = R . Furthermore let’s define the saturation voltages VH and VL which we take to be symmetric about zero: VH = −VL . In order to proceed we must assume an initial state for the output voltage Vo . Let’s start with Vo = VL which gives 1 (1.21) V+ = VL 2 Recall that we are operating with the positive feedback and thus the output is given by Vo = A(V+ −V− ) (1.22) 1 The voltage V+ = VL is determined by the characteristics of the capacitor C and the 2 resistor RF . The form of V− is the standard exponential function of a transient RC circuit. The voltage Vo will change state (i.e. switch from Vo = VL to Vo = VH ) when V− becomes 1 less than VL . 2 The general form of the voltage V− is given by V− = V final + (Vinitial −V final )e − t / RF C (1.23) The evolution after the first transition ( Vo going from VL to VH ) which happens when 1 1 V− = VL is found from Equation (1.23) with Vinitial = VL and V final = VH 2 2 1 V− = VH + ( VL −VH )e − t / RF C 2 (1.24) V− starts to increase exponentially towards VH . The function that describes the 1 relationship between the inputs and the output of the op-amp is Vo = A( VH −V− ) and 2 1 when V− crosses VH the output will switch from VH to VL and the process continuous 2 The time evolution of the voltages of interest are shown on Figure 22 where VH = +5V , VL = −5V and R1 = R2 . Assuming that the initially Vo = VH = +5V and 1 5 V− = VL = − Volts , the equation that describes the evolution of V− for t>0 is 2 2 3 (1.25) V− = VH − VH e− t / RF C 2 t1 t2 . Figure 22. Oscillator voltage characteristics 5 Since Vo = 5Volts , V+ = Volts and the output will change state once V− crosses 2 5 Volts . This happens at t = t1 and t1 satisfies 2 5 3 = 5 − 5e − t1 / RF C 2 2 (1.26) t1 = RF C ln(3) (1.27) And solving for t1 we obtain Due to the symmetry of the problem (VH = −VL ) the time between t1 and t2 is also given by t2 − t1 = RF C ln(3) (1.28) And so the period of the square wave is T = 2RF C ln(3) (1.29) An additional consequence of the frequency is that the duty cycle1 of this square wave is 50%. If a higher (>50%) duty cycle is desired then VH > VL The general expression for the period of the square wave is R1 ⎞ ⎛ T = 2 RF C ln ⎜1 + 2 ⎟ R2 ⎠ ⎝ (1.30) The proof of Equation (1.30) follows from the analysis above and it is left as an exercise for the student. Note that the period of the square wave does not depend on the op-amp characteristics but only on the external components used in building the circuit. The quality of the square wave as the frequency increases is however dependent on the frequency characteristics of the op-amp. 1 duty cycle = t1 t2