Open-Loop Gain & Frequency


Unlike the ideal op-amp (on the bottom), the op-amp that is used in more realistic circuits today, does not have infinite gain and bandwidth. Look at Open-loop gain in above, it is graphed for a type 741 op-amp as a function of frequency. At very low frequencies, the open-loop gain of an op-amp is constant, but starts to taper off at about 6Hz or so at a rate of -6dB/octave or -20dB/decade (an octave is a doubling in frequency, and a decade is a ten-fold increase in frequency).

This decrease continues until the gain is unity, or 0 dB. The frequency at which the gain is unity is called the unity gain frequency or fT. Maybe the first factor in the consideration of a specific op-amp is its "gain-bandwidth product" or GBP. For the response curve of Fig. 4, the product of the open-loop gain and frequency is a constant at any point on the curve, so that:

GBP = AolBW Graphically, the bandwidth is the point at which the closed-loop gain curve intersects the open-loop curve, as shown in figure for a family of closed-loop gains. For a more practical design situation, the actual design of an op-amp circuit should be approximately 1/10 to 1/20 of the open-loop gain at a given frequency. This ensures that the op-amp will function properly without distortion. As an example, using the response in Figure (bandwith for the 741) , the closed-loop gain at 10Khz should be about 5 to 10, since the open-loop gain is 100 (40dB).

One additional parameter is worth mentioning, the Transient Response, or rise time is the time that it takes for the output signal to go from 10% to 90% of its final value when a step-function pulse is used as an input signal, and is specified under close-loop condistions. From electronic circuit theory, the rise time is related to the bandwidth of the op-amp by the relation: BW = 0.35 / rise time

 Open-Loop Gain: Lets have a look how the ´ideal´ amplifier would look like in Figure on the butoom. The search for an ideal amplifier is, of course, a futile exercise. The characteristics of the operational amplifier are good enough, however, to allow us to treat it as ideal. Below are some amplifier properties that make this so. (Please realize that these ratings are next to impossible to achieve). 1. Gaininfinite 2. Input impedanceinfinite 3. Output impedancezero 4. Bandwidthinfinite 5. Voltage outzero (when voltages into each other are equal) 6. Current entering the amp at either terminalextremely small.

Power Supply: In general op-amps are designed to be powered from a dual or bipolar voltage supply which is typically in the range of +5V to +15Vdc with respect to ground, and another supply voltage of -5V to - 15Vdc with respect to ground, as shown in Fig. 7. Although in certain cases an op-amp, like the LM3900 and called a ´Norton Op-Amp´, may be powered from a single supply voltage.

Electrical Ratings: Electrical characteristics for op-amps are usually specified for a certain (given) supply voltage and ambient temperature. Also, other factors may play an important role such as certain load and/or source resistance. In general, all parameters have a typical minimum/maximum value in most cases.


Keywords : 741, Open, Gain, Opamp, Operational Amplifiers, Amp, Amplifier, Diagram, Schematic
Writer : delon  |
16 Dec 2005 Fri   
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