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Operational Amplifier Ä It is a three-terminal device with two inputs, one inverting (- symbol) and the other non-inverting (+ symbol). The output signal of the op-amp is given by the difference between the voltages applied to the two inputs, multiplied by the gain A, of the op-amp. v1 i1 = 0 + A(v1-v2) v2 i2 = 0 vo + - - Features of an “ideal” op-amp: The input resistance is infinite so that no current flows into either input of the Circuit diagram of an “ideal” op-amp op-amp. (note that on the equivalent circuit they are shown as not connected to anything inside the op-amp!) r r The output resistance is zero. I.e., the op-amp can drive any load to any voltage. r The open-loop gain (A) is infinite. r The output voltage is zero when the input voltage difference is zero. r The bandwidth is infinite. However, there aren’t many truly ideal things around! Sure I’m not perfect, but what do you expect for $0.80? Every thing?! + - What would op-amps be good for? • • • • • • • • amplifiers, integrators, differentiators, comparators, precision rectifiers, oscillators, filters, and a whole mess of other things! Op-amps with feedback: a a + Positive feedback, is not great for amplifiers, but good for oscillators. - Negative feedback is great (not always the case when used with people)! It is used in most amplifiers and improves your circuit in several ways. + Finally what can you do with Op-amps? Voltage Follower It is the simplest op-amp circuit. It is used where an identical “copy” of an input signal is desired. V- Vout Vin + - V+ + As for all op-amp circuits using negative feedback, the circuit automatically keeps the voltage difference between the two inputs at zero (or very nearly so!). Clearly V- = Vout A The basic op-amp equation: Vout = A.(V+ - V - ) => Vout = A + 1V+ Since op-amp’s open-loop gain is very large (infinity for the ideal op-amp), the output voltage thus equals the input voltage. The Inverting Amplifier The next op-amp configuration is the inverting amplifier, which produces an inverted output with respect to the input signal. The output signal is also amplified by a factor that is determined by the ratio of the the two resistors (some gain at last!). ifb R2 R1 Vin + - V- iin V+ V+ = V− = 0 Vout + To see why the V- input must be at or near ground in this case, consider that V o u t = A (V + − V − ) ⇒ V − = − ∴ V − → 0 as A → ∞ iin = i fb V Vin and i fb = − out R2 R1 V V R2 Vin or in = − out ⇒ V out = − R1 R2 R1 iin = V out A The Non-Inverting Amplifier i2 R2 R1 V- i1 Vin + - V+ Vout + V− = V+ = Vin Since no current flows into an ideal op - amp, i1 = i2 or Vout − V− V− V R2 = ⇒ out = 1 + R2 R1 Vin R1 BASIC PARAMETERS OF OP-AMPS Offset voltage- If an op-amp were perfectly balanced, the DC output voltage, Vo, would be zero when no differential voltage is applied to the inputs. owing to minor unbalances, this is not the case. A real op-amp can be thought of as a perfect op-amp with a small offset voltage, Voff, applied differentially to the inputs.Owing to the large voltage gain, even a small Voff can result in a large Vout. Many op-amps have two pins to which an external potentiometer connected to a DC voltage can be connected to “null” out Vout. Slew Rate (SR)- Since op-amps are not infinitely fast, their gain decreases as the input frequency increases. This makes sense, since for a higher frequency, the transistors inside must swing the output faster and faster to reach the same output amplitude. This brings us to the term slew rate, which specifies the maximum rate at which the op-amp can swing its output. Slew rate is traditionally given in units of V/ µs. Bandwidth- A very important fact is that the product of the gain and the frequency is constant at any point on the response curve of the amplifier. That means that you are always trading off gain for frequency response. The more gain, the sooner the response begins to roll off. The gain-bandwidth product is a key parameter in the selection of op-amps since it expresses the limits for amplification / frequency response performance. Common-Mode Signal Rejection- If op-amps were perfect difference amplifiers, then the output should always be zero if you apply the same signal to the non-inverting and inverting inputs at the same time. Well, op-amps are not perfect at this either! Such a signal, applied to both inputs at the same time, is called, a common-mode signal. Common-mode rejection is important to consider because often external noise (such as 60 Hz hum) is applied to op-amps unintentionally. Output Voltage Swing- While the op-amp can swing its output nearly to the supply voltages (usually +/- 12 or 15 volts), it obviously cannot swing them farther. If an input signal is so large that the op-amp circuit’s gain calls for a swing beyond the supply voltages, the amplifier’s output will clip. In another word, for an op-amp to function linearly, the output voltage and output current must be less than saturation voltage and saturation current. This is the effect that causes the nasty audio distortion when you turn an amplifier up too loud. (Attention for rock & roller fans!) Other Op-Amp Parameters- There are some other op-amp parameters that are often considered (but will not discussed here), such as noise performance of the op-amp, stability, its power consumption, its input resistance (sorry folks, it is not infinity!) and still some other parameters. The interested student (is there any such thing?!) can inquire more information about op-amps from the references that have been referred in the text book. If you are even more interested you may want to do a bonus project designing a wave generator with op-amps.