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Op Amp Voltage = Amplifiers
=20 =20- After studying this section, you = should be able=20 to:
- Compare ideal and practical op amps.
- Understand the operation of op amps
- =E2=80=A2 The use of negative feedback.
- =E2=80=A2 Current rule for op amps.
- =E2=80=A2 Voltage rule for op amps.
- =E2=80=A2 The inverting voltage amplifier.
- =E2=80=A2 The non-inverting voltage = amplifier.
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Fig. 6.7.1 The Ideal Op Amp
The Ideal Op Amp
Although the ideal op amp does not exist in reality, a real = (practical) op=20 amp comes reasonably close to achieving the ideal. An ideal amplifier = should, in=20 addition to other desirable parameters, have at least an infinitely high = input=20 impedance, an output impedance of zero ohms, an infinitely high gain and = an=20 infinitely wide bandwidth. Table 1 compares some imprtant parameters = such as=20 Input Impedance(ZIN), Input Bias Current = (IIN),Large=20 Signal Voltage Gain (AV) and Output Impedance = (ZOUT) of=20 some typical real (practical) op amps with the =E2=80=98ideal=E2=80=99 = op-amp model:
| Table 1: The Ideal Amplifier v The = Practical=20 Op Amp | ||||
|---|---|---|---|---|
| ZIN | IIN | AV | ZOUT | |
| The Ideal Op-Amp | Infinity. | Zero. | Infinity | 0=CE=A9 |
| 741 | 2M=CE=A9 | 80nA | 316 to 200,000 (50dB to 106dB) | Depends on gain and feedback but typically less = than 100=CE=A9=20 to more than 1K=CE=A9 |
| TLC271 | 1T=CE=A9 | 60pA | 5,000 to 46,000 (74dB to 93dB) min. | |
| LMC660 | >1T=CE=A9 | 0.002pA | 40,000 to 990,000 (92dB to 126dB) |
Negative Feedback
There are two basic methods of connection for op amp voltage = amplifiers,=20 making the op amp into an inverting or a non-inverting voltage = amplifier. In=20 each case, the voltage gain of the amplifier is set simply by the ratio = of two=20 resistors. Using a very high gain amplifier and applying negative = feedback=20 produces a very stable amplifier with a gain=20 that is is virtually independent of changes in temperature or variations = in=20 semiconductor characteristics. Just as in the discrete component = amplifiers=20 described in Amplifiers=20 Module 3, the negative feedback also reduces distortion and noise in = addition to increasing the bandwidth of the amplifier.
Op Amp Rules
Ideal op amps, when used with feedback, operate in a way that can be=20 predicted by a couple of basic rules, often called "Golden Rules".
Rule 1. The Voltage Rule.
The op amp output will change as necessary to keep the two input = voltages=20 identical. If any input signal or voltage tries to make the input = potentials=20 different, the op amp output will change in the opposite polarity to the = input=20 and, via the feedback loop, keep the difference between the two inputs = at=20 0V.
Rule 2. The Current Rule.
Because the input impedance is infinitely high, no current can flow = into=20 either input.
These rules used in relation to the ideal op amp can be used to make = the=20 operation of the two main voltage amplifier circuits easier to = understand. The=20 small differences between ideal and practical op amps can be = temporarily=20 ignored.
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Fig.6.7.3 The Inverting Amplifier
The Inverting Amplifier
Fig. 6.7.3 shows the inverting amplifier, in this configuration the = signal=20 input is applied to the inverting (=E2=88=92) input to produce an = anti-phase output=20 signal whose amplitude is Vin x Avcl where = Avcl=20 is the closed loop gain of the op amp.
Negative feedback is used to reduce the op amp=E2=80=99s very high = maximum gain to=20 the required level. The closed loop gain (Avcl) is set simply = by the=20 ratio of Rf to Rin
How the Inverting Amplifier Works
By applying the two rules to the ideal op amp shown in Fig. 6.7.3a it = can be=20 assumed that:
Pin 3 is at 0V, as it is connected to ground via R3, which will have = no=20 voltage developed across it as no current flows into pin 3 (Rule 2), so = pin 2=20 will also be at 0V (Rule 1).
Therefore Rin and Rf are effectively connected = in=20 series between Vin and Vout with pin 2 between the = two=20 resistors held at 0V.
No current can flow into pin 2 (Rule 2) so the current flowing into=20 Vin cannot be flowing into the op amp, therefore it must be = flowing=20 through Rf to the output.
The diagram in Fig. 6.7.3b shows that in effect Rin and=20 Rf are two resistors connected in series between = Vin and=20 Vout with Pin 2 at 0V.
Although pin 2 is not actually connected to ground, it must be at = the same=20 voltage as pin 3 (Rule 1) which is at 0V (because of Rule 2). This = important=20 effect of making some location in a circuit that is not actually = connected to=20 ground, yet maintains a potential of 0V is called =E2=80=98virtual = earth=E2=80=99 (or virtual=20 ground).
Any input current (Iin) therefore flows directly from the = input,=20 via Rin and Rf to the output, with Rin = and=20 Rf forming a potential divider between the opposite polarity = voltages=20 Vin and Vout with pin 2 at 0V.
The current through a series=20 resistor circuit is the same for each resistor, so the input voltage = Vin will be proportional to the resistance of Rin = and the=20 voltage across Rf will be proportional to the resistance of=20 Rf
Notice that because = Rf is=20 effectively connected between the output terminal and virtual earth (0V) = the=20 voltage across Rf is also equal to Vout. This = makes=20 Rf / Rin equal to Vout / Vin = (the=20 gain of the amplifier) and therefore the closed loop gain of the = inverting op=20 amp voltage amplifier (Avcl) is given by the equation:
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Notice that the formula only tells you the ratio of the resistors = and not=20 their actual values. Fortunately in audio amplifiers the values are not = too=20 critical, usually keeping the resistor values between about 10K and 100K = is OK.=20 However it is good to try and keep Rin as high a resistance = as=20 possible, and also in a practical amplifier, rather that grounding the=20 non-inverting input directly it should be grounded by a resistor with = the same=20 value as Rin to keep the (tiny) input currents equal. This = gives a=20 better chance of the output voltage being zero volts (or close to it) = when the=20 input is zero volts.
=20The Non-inverting Amplifier
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Fig.6.7.4 The Non-Inverting Amplifier
In the non-inverting amplifier illustrated in Fig. 6.7.4a the input = is=20 applied to the non-inverting input (pin 3) while the negative feedback = resistor=20 (Rf) together with R1 set the closed loop gain of the = amplifier.
The input and output signals are now in phase, which changes the = circuit=20 operation. As shown in Fig. 6.7.4b, the circuit does not have a virtual = earth=20 point, but the bottom end of R1 is connected to ground, which means that = pin 2=20 will follow the varying input voltage Vin on pin 3 (Rule = 1).
Rf and R1 now form a potential divider between = Vout and=20 0V. Just like the inverting amplifier, no current will flow into pin 2 = (Rule 2)=20 so the voltages across R1 and Rf will be proportional to = their=20 individual resistances. The ratio of these resistances, and therefore = the ratio=20 of Vout to Vin i.e. the closed loop gain is given = by the=20 standard potential=20 divider formula (Rf + R1) / R1.
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