1. The use of Op-Amp in digital systems is found in the circuit:
- Active speaker
- Summing amplifier
- Audio mixer
- Difference amplifier
- Instrumentation amplifier
- Encoder and Decoder
Op-Amp
2. The use of Op-Amp in analog systems is found in the circuit:
- Analog to digital converter ADC (Analog to Digital Converter)
- Digital to Analog Converter (DAC)
- Comparator circuit
- Pulse width modulator
3. Op-Amp Characteristics:
- Very high input impedance
- Very low output impedance
- High voltage gain reaches 105 times
- The supply voltage provided is symmetrical DC
- DC and AC input voltage signals
- Input stationary current in the order of milliamperes
Op-Amp input consists of an inverting input with the symbol (-) and a non-inverting input (+). In the inverting amplifier, the input signal is connected to the inverting input, so the output signal will be in the opposite phase to the input signal, conversely, in the non-inverting amplifier, the input signal is connected to the non-inverting input, so the output signal will be in phase with the input signal.
741 Op-Amp Architecture
Inverting (Inverting Amplifier)
1. Open loop or no feedback
Open loop inverting
2. Closed loop or with feedback
Closed loop inverting
Non Inverting (Non-inverting Amplifier)
1. Open loop or no feedback
Non inverting open loop
2. Closed loop or with feedback
Non inverting closed loop
Working Principle of Op-Amp Amplifier
Basically an IC op-amp is a dc coupled differential amplifier with a very large gain. The symbol in Figure 6.106 shows the availability of two input terminals. The first terminal is called the non-inverting input, marked + , the second terminal is the inverting terminal, marked -. The open-loop voltage gain Avol is 100 dB (100,000 in voltage ratio), so only a small differential input is needed to get a large input change. What is meant by differential is a signal that causes a fractional difference of 1 millivolt between the two input connections. For example, if the inverting input is 0 volts and the non-inverting input level is made + 0.1 mV; then the output will be positive approaching + 10V. If the non-inverting input level is made - 0.1 mV; the output will be -10 V. Similarly if the non inverting input is 0 volts and the inverting input is made +0.1 mV, the output will be -10 V. The amplifier responds to the voltage difference between the two inputs and if this difference is zero, the output should also be close to zero. So the op-amp must be provided with positive and negative voltage supply voltages, so that the output can swing around zero.
The transfer characteristic is shown in Figure 5.2.b. This figure shows that, if (V1 - V2) is positive, the output will also be positive. The output will saturate when (V1 - V2) reaches about +0.1 mV. Likewise, if (VI - V2) is negative, the output will be negative. This characteristic has been described by zero at the point where V1 = V2. In practice, there is always an offset, and for that it is necessary to add a potentiometer to "trim out" or nullify any off-set voltage. This will be discussed later. One measure of the quality of an op-amp is the CMRR (Common Mode Rejection Ratio). Where:
CMRR = differential gain / common mode gain
The main advantage of the differential arrangement is that if signals of the same polarity are applied to both inputs, they effectively cancel each other out and the output will be very small. Such signals are called "common mode".
Op-amps with high CMRR can be used to measure small differential signals that accompany a common mode signal as large as in the case of electrodiagram signals originating from these two electrodes having amplitudes of about 1 mV, but nevertheless these two electrodes may contain a common mode signal that is usually about 0.1 V at the power line frequency. Op-amps with high CMRR approximate and amplify the differential signal and discard the common mode signal.
Since the closed-loop gain depends solely on the values of the feedback loop components, and since these can be made with small tolerance resistors, the gain of the amplifier system can be controlled accurately. The application of negative feedback is shown in Figure 6.107.as/d d. Four important circuits are shown here, and the others are developments of these circuits.
Figure 6.106: Op-amp Symbol and Its Displacement Characteristics
Figure 6.107: Methods for Applying Negative Feedback to an Op-Amp
The main performance characteristics of an op-amp are:
- AVOL open loop voltage gain: low frequency differential gain without any feedback applied.
- Input resistance Rin: resistance attached directly to the input terminals in open loop condition. Value for bipolar IC is 1Mohm then for FET input stage may be greater than 1021 ohm.
- Input off-set voltage: for inputs that are both grounded, ideally the output of the op-amp should be zero. However, due to slight voltage inaccuracies in the input circuit, an off-set voltage can occur. The value of this differential input off-set is about 1 MV. Most modern op-amps are equipped with a means to make this off-set zero.
- CMRR: the ratio between the differential amplifier and the common mode amplifier, namely the amplifier's ability to reject common mode signals.
- Supply Voltage Rejection ratio: if a sudden power input (step) is applied to an op-amp, the output will not be able to respond quickly. However, the output will shift to a new value at a uniform rate. This is called slew rate limiting, which affects the maximum rate of change of voltage at the output of the device. This slew rate varies between 1 volt/u sec (741) to 35 volts/sec (signetic NE 531 see figure 6.108.).
- Full power bandwidth: the maximum signal frequency at which the full output voltage swing can be found.
- Full voltage swing: the peak output swing, referenced to zero, that can be found.
Figure 6.108: Op-Amp Slew Rate Limiting
Some Op-Amps available on the market are given parameters as in table 6.5.
Table 6.5: Op-Amp Parameters and Their Characteristics
The 709 also requires external components to provide frequency compensation and to prevent unwanted oscillations.
Most of these problems have been overcome in later generations of IC op-amp designs. The 741 and NE 531 are short-circuit protected and provide zero offset voltage capability and do not have latch-up problems.
The frequency response for the 741 op-amp is shown in Figure 6.109. It can be seen that at 10 kHz, the open-loop gain drops to 40 dB (100 being a voltage ratio) and at 100 kHz the open-loop gain drops to 20 dB.
Figure 6.109: Frequency Response of Op-Amp 741
The 741 has internal frequency compensation components to prevent unwanted oscillations, and this causes the gain to be reduced. If a wider power bandwidth is required, the Motorola MC 1741S or the Silicon General SG 471S can be used, which have a full power bandwidth of 200 kHz.
Cases In Op-Amp Circuits
Given two cases of circuits using Op-Amp below:
1. Square Wave Generator (see figure 6.110):
Op-Amp can be used as a square wave generator because it has a very high open-loop gain value and the availability of differential inputs. When a power supply is used in the circuit, and the capacitor C has not been charged, the Op-Amp output will saturate at its positive saturation level (Vsat+). Part of this output voltage will be fed back to the non-inverting input through R2 and R1. The voltage at the non-inverting input will be:
As long as the voltage at the inverting terminal is less than V+. Then the output will remain at a positive saturation level. However, charging C through R will cause an increase in the voltage at the inverting terminal. if the voltage becomes greater than the voltage level at the non-inverting terminal, the Op-Amp output will change to a negative saturation voltage (Vsat -). The voltage at the non-inverting terminal is now of opposite polarity and becomes:
Figure 6.110: Square Wave Generator
Now the capacitor is discharged through R, until its voltage drops to Vsat-. When the capacitor voltage at the non-inverting terminal is the same as the voltage at the inverting terminal, the Op-Amp output will return to the positive level again. This will happen repeatedly so that this circuit will produce a square wave. RC will determine the frequency of the wave produced, while R1 and R2 will determine the switching point (from Vsat+ to Vsat- or vice versa). Changes in SW1 and RV1 determine the magnitude of the frequency apart from R1 and R2, formulated as follows:
From the results of the calculations and circuit tests, the following frequencies will be obtained (minimum and maximum RV1 conditions):
Meanwhile, RV2 is used to change the mark-to-space ratio (the ratio of the positive pulse size to the pulse period) or in digital it is known as the duty cycle.
The case of the above series is:
- There is no oscillation at the output, there is only a positive saturation voltage = 8 Volts. The answer: The circuit does not oscillate because R or C is open, and because the saturation condition is + then the 3rd leg of the IC gets a large input continuously, so there is an open RV1 leg towards R1.
- There is no oscillation at the output, there is only a negative saturation voltage = - 8Volt. The answer: Same as the first case only the middle leg of RV1 is open now, so that leg 3 of the IC does not get any input, so the output must be negative.
- The change in RV2 causes a large frequency change in each interval, but only a small change in the mark-to-space ratio. The answer is: RV2 should not affect the frequency change when it is normal, and the work of RV2 is assisted by D1/D2 and R3 and R4 when charging and discharging the capacitor. Because it still works even though its function changes, but there is no open circuit. So there must be a short circuit, and of course it must be D3 or D4 that is short circuited.
- If R2 changes to a large value, then the frequencies will remain large in each interval.
2. Low Frequency Function Generator:
Function generator is an oscillator that produces simultaneously triangle, square and sine waves (see figure 6.111). This circuit uses two Op-Amp, which produce low frequency output. IC1 is connected with C1 as an integrator, and IC1 as a comparator circuit. If the output of IC2 is positive towards the positive saturation level output. The positive level section will appear at measurement point 2 (TP2) because it is a voltage divider built by R4 and R5. If R5 is 1K8 then the level at TP2 is around +700mV. Because the non-inverting input of IC1 is connected to ground, the inverting input should also be close to ground. Therefore, C1 will be charged through R1 with a current of about 10 ?A. The output of IC1 becomes negative as C1 is charged and because the charging current through R1 is almost constant, the value of the change in the output of IC1 is linear.
Figure 6.111: Low Frequency Generator Function
When the voltage at measurement point 1 (TP1), the output of IC1 exceeds a level sufficient to cause pin3 of IC2 to go below zero, the output of IC2 will be negative. Note that IC2 has positive feedback through R3, so when pin 3 is more positive than pin 2 the output will be positive, but when pin 3 is more negative than pin 2 the output will be negative.
Because the Op-Amp gain is 100,000 the change action becomes very fast. The level at measurement point 1 (TP1) which triggers the comparator IC2 is determined by R3 and R2. Because the IC2 output is a positive saturation voltage of about +4V, when TP1 is about -2V pin 3 will go below zero and the IC2 output will turn negative.
With the output of IC2 at -4V, TP2 also turns negative to -700mV. The charging current for C1 is now reversed and TP1 becomes positive. When the level at TP1 reaches about +2V, the comparator changes again and the process repeats. The time for C1 to charge from -2V to +2V is the time for one half of the oscillator wave. To get an approximate value for this time, we can use the formula:
Q = CV
If the capacitor is charged with a constant current
With C = 1uF, I = 10uA and dV = change in voltage that occurs across the capacitor = 4 Volts.
The actual frequency of operation depends on several factors such as IC2 saturation voltage, C1 tolerance and resistor tolerance. By making R5 preset the frequency can be set to 1Hz.
The triangle output is converted to a sine wave by diodes D1, D2, D3, D4. R6 and R7 act as a voltage divider that can result in the output through R7 being 3 Vpp. However, the diode conducts when forward biased with 500mV and produces a sine wave with an amplitude of 2Vpp. This is a triangle to sine converter and produces a rather high distortion. R5 can be adjusted to obtain optimal results.
The case of the above circuit is:
- The frequency of the circuit will go high about 66.5 Hz and the frequency cannot be adjusted. The answer: The one that regulates the frequency is R5, so if the frequency cannot be adjusted by it then of course the middle leg of R5 is open, so the frequency is still there.
- There is distortion in the positive sine wave, while the other waves are normal. The answer: the sine wave occurs because of the diodes and R6 and R7. Because the result is a defect in the positive part, it means that there is something wrong with the voltage divider, namely R7 is open.
- There is a wave as described below on the sine output, the other outputs are fine. The answer: what causes the positive part of the sine wave to be damaged must be the diode whose anode is pointing to the more positive one, so D3 or D4 must be open.
If D1 is shorted, the output sine wave will be distorted, and the waveform will be close to only ½ positive wave.
Is Our Op-Amp Configuration Correct?
This question we raised from a member who consulted via Facebook. Actually the question is good, it's just that the place is not appropriate, because he asked in a post that the discussion is 180^ different from what he asked. The comparison is website content vs. electro.
Well, it doesn't need to be long, so this is what it sounds like:
Assalamualaikum, peace be upon you, sir. I want to ask, if I use IC 741, if one of the pins such as V+ or inverting is not connected to other components but is connected to ground, will the circuit work or not? Thank you.
Answer
V+ is for the working voltage supply, if it is not met then the device will not function / will not work.
Meanwhile, inverting and non-inverting inputs are only optional, the signal that you want to amplify and invert or not, so please adjust it according to your needs.
User [ask]
I made a circuit schematic like this, sir, the touch alarm uses a touch plate to turn the LED and BUZZER on and off, but in this schematic I use logic input.
and I have simulated the circuit using livewire software and it works, but in your opinion, when my circuit is applied to the PCB, will there be any problems or not? especially on the IC?
Thank you for your response, sir.
Answer
Yes sir, that circuit means you are using the inverting feature, as I mentioned earlier, but for the speaker, don't parallel it with vcc, because it can be damaged if DC is supplied. Ideally, the speaker is given a buffer first (Elco as a filter to prevent DC and pass AC).
User [response]
Wow, it turns out to be true, bro, when I paralleled the speaker with the VCC, it immediately gave off an unpleasant smell, haha, thanks for your input, bro, I hope your knowledge will continue to grow, bro. Amen.
Getting to Know Op-Amp as an Active Filter
1). Inverting (Inverting Amplifier)
- Open loop (no feedback)
- Closed loop (with feedback)
2). Non-Inverting (Non-Inverting Amplifier)
- Open loop (no feedback)
- Closed loop (with feedback)
3). Low Pass Filter (LPF)
4). High Pass Filter (HPF)
5). Band Pass Filter (BPF)
6). Tape Recorder
