Here, a fixed reference voltage is applied to the inverting terminal and a variable test or sample voltage is fed to the non-inverting terminal. One special application of the 'open-loop' op-amp is as a differential voltage comparator, one version of which is shown in Figure 4(a). The voltage gains of the Figure 3 circuits depend on the individual op-amp open-loop voltage gains, and these are subject to wide variations between individual devices. Methods of using the op-amp as a high gain, open loop, linear DC amplifier. Note in the latter case that if identical signals are fed to both input terminals, the op-amp should - ideally - give zero signal output.įIGURE 3. Alternatively, it can be used as a non-inverting DC amplifier by reversing the two input connections, as shown in Figure 3(b), or as a differential DC amplifier by feeding the two input signals to the op-amp as shown in Figure 3(c). Thus, an op-amp can be used as a high-gain inverting DC amplifier by grounding its non-inverting terminal and feeding the input signal to the inverting terminal, as shown in Figure 3(a). Where A o is the low frequency open-loop voltage gain of the op-amp (typically 100dB, or x100,000, e 1 is the signal voltage at the non-inverting input terminal, and e 2 is the signal voltage at the inverting input terminal). The output signal of an op-amp is proportional to the differential signal voltage between its two input terminals and, at low audio frequencies, is given by: They can, however, also be powered from single-ended supplies, if required. They are normally powered from split supplies, as shown in Figure 2(b), providing positive, negative, and common (zero volt) supply rails, enabling the op-amp output to swing either side of the zero volts value and to be set to zero when the differential input voltage is zero. Basic symbol (a) and supply connections (b) of an op-amp.Ĭonventional op-amps are represented by the standard symbol shown in Figure 2(a). The output stage takes the form of a complementary emitter follower, and gives a low-impedance output.įIGURE 2. The output of the differential amplifier is fed to the circuit's output stage via an offset compensation network which - when the op-amp is suitably powered - causes the op-amp output to center on zero volts when both input terminals are tied to zero volts. It also has a high-impedance collector (or drain) load, to give a large amount of signal-voltage gain (typically about 100dB).įIGURE 1. The differential amplifier has inverting and non-inverting input terminals, and has a high-impedance (constant-current) tail to give a high input impedance and good common-mode signal rejection. All of these elements are integrated on a single chip and housed in an IC package. In its simplest form, a conventional op-amp consists of a differential amplifier (bipolar or FET) followed by offset compensation and output stages, as shown in Figure 1.
The other two basic types of op-amps are the current-differencing or Norton op-amp (typified by the LM3900), and the operational transconductance amplifier or OTA (typified by the CA3080 and LM13700) these two devices will be described in some future articles. The most important of these is the conventional 'voltage-in, voltage-out' op-amp (typified by the popular 741 and CA3140 ICs), and this four-part mini-series takes an in-depth look at the operating principles and practical applications of this type of device. Three basic types of operational amplifiers are readily available. When coupled to suitable feedback networks, they can be used to make precision AC and DC amplifiers and filters, oscillators, level switches, and comparators, etc. A conventional op-amp (operational amplifier) can be simply described as a high-gain direct-coupled amplifier 'block' that has a single output terminal, but has both inverting and non-inverting input terminals, thus enabling the device to function as either an inverting, non-inverting, or differential amplifier.
0 kommentar(er)
