【正文】 we mean that the output voltage with respect to ground is given by the expression:Vo=A（V+ — V） （11）Figur11 An operational amplifierWhere the input V+ and V may be DC or AC signals and A is the differential gain (voltage gain). The magnitude of A is approximately ～ for DC and AC signals with frequencies less than approximately 10Hz.(The differential gain A decreases with the signal frequency and bee about unity for frequencies of 1MHz~50MHz.)Since the gain of the gain of the opamp is very hign,it is necessary to have a negative feedback from the output to the input to make the amplifier stable.(The feedback is made from the output to the inverted input so that the feedback is a negative feedback..) The name Ideal opamp is applied to similar analysis because the salient parameters of the opamp are assumed to be perfect. There is no such thing as an ideal opamp,but present day opamps e so close to ideal that Ideal opamp analysis approaches actual analysis. Opamps depart from the ideal in two ways. First, dc parameters such as input offset voltage are large enough to cause departure from the ideal. The ideal assumes that input offset voltage is zero. Second, ac parameters such as gain are a function of frequency, so they go from large values at dc to small values at high frequencies.This assumption simplifies the analysis, thus it clears the path for insight. It is so much easier to see the forest when the brush and huge trees are cleared away. Although the ideal opamp analysis makes use of perfect parameters, the analysis is often valid because some opamps approach perfection. In addition, when working at low frequencies,several kHz, the ideal opamp analysis produces accurate answers. Several assumptions have to be made before the ideal opamp analysis can proceed. First, assume that the current flow into the input leads of the opamp is zero. This assumption is almost true in FET opamps where input currents can be less than a pA, but this is not always true in bipolar highspeed opamps where tens of 181。A input currents are found.Second, the opamp gain is assumed to be infinite, hence it drives the output voltage to any value to satisfy the input conditions. This assumes that the opamp output voltage can achieve any value. In reality, saturation occurs when the output voltage es close to a power supply rail, but reality does not negate the assumption, it only bounds it. Also, implicit in the infinite gain assumption is the need for zero input signal. The gain drives the output voltage until the voltage between the input leads (the error voltage) is zero. This leads to the third assumption that the voltage between the input leads is zero. The implication of zero voltage between the input leads means that if one input is tied to a hard voltage source such as ground, then the other input is at the same potential. The current flow into the input leads is zero, so the input impedance of the opamp is infinite.Fourth, the output impedance of the ideal opamp is zero. The ideal opamp can drive any load without an output impedance dropping voltage across it. The output impedance of most opamps is a fraction of an ohm for low current flows, so this assumption is valid in most cases. Fifth, the frequency response of the ideal opamp is flat。，this means that the gain does not vary as frequency increases. By constraining the use of the opamp to the low frequencies, we make the frequency response assumption true. The internal construction of an opamp is quite plex and usually contains a large number of discrete person working with an opamp does not ordinarily need to be concerned with its internal construction. It is helpful,however,to have some general understanding of what the internal circuitry permits the user to see how the device performs and indicates some of its limitation as a functioning unit. The internal circuit of an opamp can be divided into three functional 12 shows a simplified diagram of the internal functions of an that each function is enclosed in a schematics use the triangle to denote the amplification daigram shows that the opamp has three basic amplification functions are generally called stages of stage of amplification contains one or more active device devices and all the associated ponent needed to achieve amplification. The first stage or input of an opamp is usually a differential amplifier. This amplifier has two inputs,which are labeled V+ provide high gain of the signal difference supplied to the two input and low gain for mon signal applied to both inputs simultaneously. The input impedance is hign to any applied output of the amplifier is generally two signals of equal amplitude and 180176。out of could be described as a pushpull input and output.Figure12 Opamp diagram One or more intermediate stages ot amplification follow the ditterential amplilier. Figure 1. 2 shows an opamp with only one intermediate stage. Functionally, this amplifier is designed to shift the operating point to a zero level at the output and has high current and voltage gain capabilities. Increased gain is needed to drive the output stage without loading down the input. The intermediate stage generally has two inputs and a singleended output. The output stage of an opamp has rather low output impedance and is responsible for developing the current needed to drive an external load. Its input impedance must be great enough that it does not load down the output of the intermediate amplilier. The output stage can be an emitterfollower amplitier or two transistors connected in a plementarysymmetry configuration. Voltage gain is rather low in this stage with a sizable amount of current gain. A differential amplifier is the key or operational basis of most opamps. This amplifier is best described as having two identical or balanced transistors sharing a single emitter resister. Each transistor has an input and an out