The Ideal Opamp

The Ideal Opamp

An ideal opamp has an infinitely high input impedance, and therefore needs no bias current. It is also capable of infinite gain without feedback, so there are no errors between the two inputs (i.e. Rules 1 & 2 will hold for all cases). The ideal opamp also has zero ohms output impedance, and is capable of supplying as much current as you will ever need. The ideal opamp does not exist :-(

Although it does not exist, the ideal opamp is the common model for nearly all opamp circuits, and few errors are encountered in practice as a result of designing for the ideal, and actually using a real (non-ideal) device. The tolerance of even the best resistors will ultimately limit the accuracy of any opamp circuit at low frequencies (where gain is highest).

The primary practical limitations are:

• Input Impedance - Typically from one to several hundred Megohms. FET inputs are used for very high impedance inputs
  
• Gain - 100dB at up to a few hundred Hertz is common
  
• Common Mode Input Voltage - typically limited to the supply voltages, but may be up to 0.6V greater with some designs
  
• Bandwidth - opamps with a usable high frequency limit of 1MHz at unity gain are now common
  
• Output Current - most common opamps are limited to about 20mA of output current.

There are others, such as input offset voltage and current, but we shall not concern ourselves with these parameters just yet. Power opamps may be capable of up to 10A, but these are outside the scope of this section of the article.

The use of ideal opamps is assumed for much of the following, but all are designed to function properly with real world devices. In practice the difference between an ideal opamp and the real thing are so small as to be ignored, but with one major exception - bandwidth. This is the one area where most opamps show their limitations, but once properly understood, it is quite easy to maintain a more than adequate frequency response from even basic opamps.

The common mode input voltage can be important in some applications. Ideally, an opamp only reacts to the voltage difference between its inputs. Provided this does not change, in theory, the actual voltage between the two inputs and the common (zero volt line) may be anywhere within the specified range with no change in the output voltage. In other words, the inputs can assume any voltage between the negative and positive supplies, and there will be (almost) no change at the output.

With a real (as opposed to ideal) opamp, there will be some change, and this is specified as the common mode rejection ratio. An opamp with a CMRR of 100dB (not uncommon) will ensure that the change in output voltage is 100dB less than the change of input voltage (as applied to both inputs simultaneously. Any difference between the inputs is amplified normally. CMRR is affected by the open loop gain of the opamp, so is usually worse at high frequencies.

Keywords : Ideal, Opamp, Amplification, Amplifier, Op-amp