A voltage amplifier Simple voltage amplifier model
If the input resistance of the amplifier R in were very large, the source voltage v S and the input voltage v in would be approximately equal:
By an analogous argument, it can also be seen that the desired output resistance for the amplifier R out should be very small, since for an amplifier with R out = 0, the load voltage would be
We can see that as R in approaches infinity and R out approaches zero, the ideal amplifier magnifies the source voltage by a factor A v L = A vS Thus, two desirable characteristics for a general-purpose voltage amplifier are a very large input impedance and a very small output impedance.
The ideal operational amplifier behaves very much as an ideal difference amplifier, that is, a device that amplifies the difference between two input voltages. Operational amplifiers are characterized by near-infinite input resistance and very small output resistance. As shown in Figure 8.4, the output of the op-amp is an amplified version of the difference between the voltages present at the two inputs.
The input denoted by a plus sign is called the noninverting input (or terminal), while that represented with a minus sign is termed the inverting input (or terminal). The current flowing into the input circuit of the amplifier is zero, or:
The input signal to be amplified is connected to the inverting terminal, while the noninverting terminal is grounded. Inverting amplifier
The voltage at the noninverting input v + is easily identified as zero, since it is directly connected to ground: v + = 0. The effect of the feedback connection from output to inverting input is to force the voltage at the inverting input to be equal to that at the noninverting input.
The analysis of the differential amplifier may be approached by various methods; the one we select to use at this stage consists of 1.Computing the noninverting- and inverting- terminal voltages v + and v −. 2. Equating the inverting and noninverting input voltages: v − = v +. 3. Applying KCL at the inverting node, where i 2 = −i 1.
The differential amplifier provides the ability to reject common-mode signal components (such as noise or undesired DC offsets) while amplifying the differential-mode components. To provide impedance isolation between bridge transducers and the differential amplifier stage, the signals v 1 and v 2 are amplified separately.
The effect of limiting supply voltages is that amplifiers are capable of amplifying signals only within the range of their supply voltages.
Another property of all amplifiers that may pose severe limitations to the op-amp is their finite bandwidth. Open-loop gain of practical op-amp The finite bandwidth of the practical op-amp results in a fixed gain-bandwidth product for any given amplifier.
Another limitation of practical op-amps results because even in the absence of any external inputs, it is possible that an offset voltage will be present at the input of an op-amp. Another nonideal characteristic of op-amps results from the presence of small input bias currents at the inverting and noninverting terminals.