Cmos op amp offset
It was somewhat of an improvement over even earlier op-amps (such as the uA709), but it's pretty long in the tooth. The 741 was designed in the mid-1960s, so almost 50 years ago. MOSFET amplfiers also seldom are capable of working with higher supply voltages, such as +/-15V (another frequent requirement of precision instrumentation), and if they are, they tend to cost an arm and a leg, I think that's mostly because they have to be made on a special high-voltage CMOS process line and not mixed with digital stuff. It has a lot of input bias current, but very little distortion. If you are doing audiophile audio processing, the best amplifier in the world is a Texas Instruments (nee Burr-Brown) bipolar part. (If that's not good enough, a discrete design can do better).Ĭontrast that with a typical MOSFET-input amplifier such as the MCP601, which is typically 29nV/sqrt(Hz), or about 700 times worse in terms of power. If you have a low impedance source, for the lowest noise of any available monolithic amplifier, you need to go to a bipolar amplifier such as the LT1028 which has a white noise spectral density of 1.1nV/sqrt(Hz). MOSFETs are too noisy for many precision amplifier applications. That's a tough trick for most of today's FET opamps. Also note the supply voltage range the LM324 can operate over. Nowadays, FET opamps are easier to make rail to rail for both input and output, but again compare the price of even the cheapest MCPxxxx with the old standby LM324. You can get FET input opamps with offset voltages well below a mV, but then compare their price to the LM324.Įarly FET opamps, like the TL07x and TL08x had other problems, like very high input common mode range headroom at both ends. Nowadays these distinctions are more blurred. FETs of course have the really high input impedence. There are also advantages in current drive capability and supply voltage range. In general, it's a little easier to get the offset voltage down to a few mV with bipolars for the same chip area. If the 1 MHz gain×bandwidth product, the bias current, and the few mV of offset are all good enough, then everything else is just expensive junk. Often enough you just need a opamp without very stringent requirements.
One significant advantage of the LM324 is its price. Its close derivative, the LM324, is still in volume production today, so obviously many people think it is the right tradeoff for their requirements. It's unfair to call the 741 junk it was something wonderful in its time. For one thing, they were designed many years ago when FET IC technology was less advanced relative to bipolar IC technology. Which type of op amp you use is a design consideration, factoring in speed, cost, temperature range, and any precision concerns.īipolar opamps like the 741 or the LM324 have different tradeoffs than FET opamps. For most modern op amps, these are very small currents, and can be considered negligible for the majority of applications. How much current flows into the op amp inputs depends on the input impedance. There are other op amps out there that can handle RF frequencys, or handle high output currents, but they don't really fall in these catergories.Īs you can see, each type of op amp has different non-ideal DC characteristics, and input impedance. Take a look at this article for more information It drifts very little, and has very low offsets. This is the type of op amp that can get its output within millivolts of the rails, but rail voltage is limited.Ĭhopper Stabilized - This is another form of the CMOS op amp. The 741 falls in this category.įET Inputs - These are a bit faster, have significantly better non-ideal characteristics (bias currents in the picoamps), drift very little, have extremely high input impedances (gigaohms), but may cost a few dollars.ĬMOS - CMOS op amps are slow, but have excellent non-ideal characteristics (bias currents in the FEMTOamps), extremely high input impedance (TERAohms), drift about as much as general purpose op amps, and may cost a few dollars. General Purpose - These op amps are not very fast, have bad non-ideal characteristics (bias currents in the nanoamps), drift, have input impedances in the megaohms, and cost almost nothing. Modern op amps fall into several categories. I seem to remember reading somewhere that if every 741 ever made were to be collected, there would be enough to give every person on earth 6 or 8 of them. The 741 is an old piece of junk, primarily used to teach basic electronics for cheap.