The voltage gain and input impedance are determined by the R1 and R2 values, and can be altered to suit individual needs. INVERTING AMPLIFIER CIRCUITSįigure 1 shows the practical circuit of an inverting DC amplifier with an overall voltage gain (A) of x10 (= 20dB), and with an offset nulling facility that enables the output to be set to precisely zero with zero applied input. When reading this episode, note that all practical circuits are shown designed around a standard 741-type op-amp and operated from dual 9V supplies, but that these circuits will usually work (without modification) with most voltage-differencing op-amps, and from any DC supply within that op-amp’s operating range (allowing for possible differences in the op-amp’s offset biasing networks). This installment looks at practical ways of using such op-amps in linear amplifier and active filter applications. When a voltage follower is needed, it’s almost always worth it.Our opening episode of this four-part ‘op-amp’ series described the basic operating principles of conventional voltage-differencing op-amps (typified by the 741 type) and showed some basic circuit configurations in which they can be used. There are many different op-amps out there and while a general op-amp is fine for many applications, if there are certain features that are more important than others, you can buy specialty op-amps that will trade-off certain performance characteristics for others.Įven with these drawbacks (applicable to any op-amp circuit, not just voltage followers), the issues are fairly minimal and in many cases, easily ignored or accepted. There are minute amounts of all of these. Everything that we assumed was ideal - no amplification, distortions, variations, phase shifts, and infinite input and zero output impedance - none of these are actually true. Op-amps are not perfect and their limitations may be enough to not work properly in your applications. Either failure in the design stage to catch something important, failure during the manufacturing stage when it’s being built, or failure when being used. Where there is greater complexity, there is greater chance of failure. This is related to the previous point but I consider it a standalone issue. This may not seem like such a big deal when you’re breadboarding or only making 5-10 parts, but if you’re making a design for 10,000 or 10,000,000 parts, those pennies add up very, very quickly. To use an op-amp in the circuit, you need to buy the part, allocate space for it, provide any supporting components, and make sure you have the appropriate power provided. If there’s one thing you’ll learn as an engineer, is that you never get something for nothing. To overcome the problem of changing the value while reading it, we put that high-impedance output into the op-amp’s high-impedance input, and then measure the op-amp's low-impedance output without fear of affecting the device under test. Their output is a voltage differential but they can’t source very much current, being very high-impedance, before the load causes a voltage droop (similar to the issues we talked about with voltage dividers), which makes it very difficult to get an accurate measurement of the voltage. So, a voltage follower can be set up as a buffer, changing a low impedance output to a high impedance output.Īs an example, we can think of many passive devices, such as certain thermocouples or pH sensors. An op-amp output, however, is able to sink or source quite a bit of current - they’re extremely low impedance. Op-amp inputs don’t sink or source any current - they’re extremely high impedance. It all goes back to the first of the two main rules of op-amps, as discussed in the intro to op-amps.
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