Op Amp Gain Calculator
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Op Amp Gain Calculator helps calculate the values of the output voltage and the inverting and non-inverting gains of an operational amplifier.
Introduction
The operational amplifier (op-amp) is an analog circuit module that uses differential voltage input to generate single-ended voltage output. Operational amplifiers usually have three terminals: two high-impedance input terminals and one low-impedance output terminal.
The inverting input is represented by a negative sign (-), and the non-inverting input is represented by a positive sign (+). The role of the operational amplifier is to amplify the voltage difference between the inputs, which is very useful for various analog functions such as signal chain, power supply and control applications. Under current technical conditions, the mathematical operation function of operational amplifiers is no longer prominent and is now mainly used in signal amplification and active filter design.
In most conventional designs, we use the ideal model of the op-amp, ignoring its internal structure. Think of it as an "amplifying component", connect it to a power source, and let it play an amplifying role. The so-called ideal op-amp has infinite input impedance and zero output impedance, as shown in Figure 2.1.
Ideal operational amplifier circuit analysis has two important principles throughout, namely "virtual shortness" and "virtual disconnection". "Virtual short" means that the positive terminal and the negative terminal are close to a short circuit, that is, V+=V-, which looks like a "short circuit"; "virtual break" means that the current flowing into the positive terminal and the negative terminal is close to zero, that is, I+=I- =0, it looks like an open circuit (because the input impedance is infinite).
Typical Applications of Op-amp
• Inverting Proportional Amplifier Circuit
Figure 2.2 is a typical proportional amplifying circuit. According to the "virtual short" and "virtual disconnection" rules, the result can be easily calculated:
The negative sign in Equation 2.1 means that the phase difference between the output and the input is 180°.
• Differential Amplifier Circuit
Figure 2.3 shows the differential amplifier circuit, which is a "variant" of the inverted proportional amplifier circuit in Figure 2.2. Similar to the analysis method of the inverted proportional amplifier circuit, conclusions can be drawn:
When R1=R3 and R2=R4, equation 2.5 is obtained. This is the origin of the name of this circuit, it can amplify differential signals.
• Non-inverting Amplifier Circuit
The amplifying circuit introduced above will cause the phase to be reversed by 180°. Figure 2.4 shows the non-inverting amplifying circuit. As the name implies, the output and input maintain the same phase. The ideal operational amplifier has the characteristics of infinite input impedance and infinitely small output impedance. The non-inverting amplifier circuit maintains this characteristic of the operational amplifier.
Analyzing Figure 2.4, applying the "virtual short" of the op-amp, we know that V2=V1; besides, because of the "virtual break" of the op-amp, the output voltage current all flows through R2 and R1, so V2 is divided by R1 and R2 to Vout get.
Therefore,
Adjust R2 to increase the magnification of the circuit.
Note that the application of the non-inverting amplifier circuit has limitations. Generally, it is only used for the amplification of DC level, and is not suitable for the amplification of AC signals, because it will also amplify the DC bias voltage of the AC signal to make it bias. The potential has shifted.
Advantages and Limitations of Operational Amplifier
There are many advantages to using an operational amplifier. It usually comes in the form of an IC, it is easy to buy, and countless optional performance specifications can meet all application requirements. It has a wide range of uses and is a key component of many analog applications including filter design, voltage buffers, comparator circuits and many other applications. Besides, most companies provide simulation support, such as PSPICE models, which can be used by designers to verify their operational amplifier designs before building actual designs.
The limitation is that they are analog circuits, which require designers to understand the basic principles of simulation, such as load, frequency response, and stability. It is not uncommon to design a seemingly simple op-amp circuit, but oscillations occur in the end. The designer must understand the key parameters discussed in the previous article and understand how they affect the design. This usually means that the designer must have mid-to-upper level analog design experience.
FAQ
1. What is the gain of an op-amp?
The gain of an op-amp signifies how much greater in magnitude the output voltage will be than the input. For example, an op-amp with a resistor, RIN, of 1KΩ and a resistor, RF of 10KΩ, will have a gain of 10. This means that the output will be ten times greater in magnitude than the input voltage. So, for example, if the input voltage is 5V in magnitude, the output voltage will be 50V in magnitude.
2. Why do op-amps gain decrease at a high frequency?
Every amplifier's gain decreases with frequency.
However, the Opamp gain is DELIBERATELY designed to roll off to 1 or less at a substantially lower frequency. The main reason is that all opamps are meant to use feedback in the circuits they are to be used. And any amplifier whose negative feedback has a phase difference of 180 degrees and a gain of more than 1, will become an oscillator, and seize to function as an amplifier or the opamp.
Therefore to make them unconditionally stable ie. Irrespective of load and source impedances, they are deliberately given a roll-off gain reduction to 1 much before their output phase lag reaches the 180-degree point. Some manufacturers also state this in their datasheets under the term, phase margin. Phase margin is the number of degrees by which an output of an opamp lags behind the input, at the frequency at which the opamp open-loop gain drops to 1.
3. Why is it necessary to reduce the gain of an op-amp from its open-loop value?
The open-loop gain of most op-amps starts to fall off at 10 Hz in order to make them stable in unity gain applications. Op-amps are designed to be rather foolproof, short protected and other mistake forgiving. In audio design we need lots more bandwidth, say to 50 kHz because bandwidth is measured to 3 dB down so we don't want to be down much at 20 kHz.
Therefore to expand the frequency response gain must be reduced. You can use the gain-bandwidth product to predict the gain at high frequencies. Typical op-amps have about 1 MHz gain bandwidth. Therefore we can get a gain of 20 out of such an op-amp. There are op-amps with wider bandwidth and some with external compensation so you can get more bandwidth if you have less feedback.
4. What is the difference between an open-loop gain and a closed-loop gain in an OP-AMP?
The main and only difference is whether the output is feedback to the input or not. Open-loop is also known as without feedback and Closed-loop is with feedback.
Comparing the two:
• The voltage gain of the op-amp is very resulting in the clipping of the output in an open-loop configuration.
• In open-loop configuration only smaller signal having low frequency may be amplified without distortion
• Open-loop voltage gain is not a constant and fluctuates with change in temperature and sometimes variation in voltage gain is very large making it unsuitable for many linear applications.
• Bandwidth is very low (almost zero) for open-loop configuration.
All of the above can be corrected using an op-amp with a closed-loop (i.e. with feedback).
5. Why is open-loop gain in op-amps very high?
An ideal op-amp has an open-loop gain of infinite. When you analyze an op-amp circuit by the usual shortcut methods (assume the inverting and non-inverting inputs are at equal voltages, assume no current flows into the inputs), those shortcut methods are only 100% accurate if the open-loop gain is infinite -- they are built on the assumption that the open-loop gain is infinite.
When the open-loop gain is not infinite, there's some small amount of current that flows into the input terminals, and there's a small voltage difference between the inverting and non-inverting inputs.
If you don't believe me, take a look at the "2nd order" op-amp model -- one that approaches ideal but allows for some non-ideal characteristics, such as finite open-loop gain and finite input impedance.
Put feedback around it and solve it. You'll see exactly what I'm saying.
Op-amps are designed to come as close to the ideal case as possible within application limits.
6. Why should op-amp have low output impedance?
Because we want to 'transfer' all the output voltage to the load. The load could be anything the op-amp is driving: a speaker, hard disk motor, digital circuits, etc. If the impedance was not 'zero', then we would see voltage division at the output and the loads will get 'weak' input.
7. Why should op-amp have high input impedance?
Because we want to 'receive' all the input voltage from the previous circuit. Imagine there was another op-amp driving this op-amp, and this one becomes the load to the previous one.
8. What is the use of the unity gain in Op-amps?
They are called buffers or voltage followers. It's desired for an amplifier that input impedance will be very high and output impedance is very low. Suppose a signal is to be fed to an amplifier. If it is fed through a buffer, it will offer a high input impedance and low output impedance. Also, suppose the amplifier in the next stage requires a good amount of current which the signal source can't supply. If connected through the buffer, the buffer can supply the power from its power source.
9. Is a unity gain inverting op-amp the same as a logical inverter?
No! A unity gain inverting op-amp output is the negative of the input linear signal, if the input is a logic level "one" the output must be the negative voltage of the range of voltage corresponding to logic levels range that always are positive (including zero on CMOS family).
The op-amp is an analog signal device, an amplifier a continuous linear function, a logical inverter is a nonlinear device that discriminates input voltage range as windows corresponding to 0 level or 1 logic level and outputs the complementary level range (even with better tolerance).
10. Which is better for amplifying gain, a transistor or an OP-AMP?
It depends on the application. Operational amplifiers (Op-amps) are voltage multipliers, while transistors, including MOSFETs (Metal Oxide Field Effect Transistors), are current amplifiers. In my experience, Op-amps driving Mosfets are the near-ideal way of using a voltage change to produce a current change. This being true as a result of high impedance MOSFET gates presenting zero loads to Op-amp outputs.
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