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Amplifier Basics

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6. Operational Amplifiers

6.0
Introduction

6.1
Op amp Inputs

6.2
Comparators

6.3 Voltage Amplifiers

6.4
Op amp Characteristics

6.5
Op amp Packages

6.6
Op amp Circuits

6.7
Op amps Quiz

Op Amp Voltage Amplifiers

What you´ll learn in Module 6.3

After studying this section, you should be able
to:

Compare ideal and practical op amps.

Understand the operation of op amps

• The use of negative feedback.

• Current rule for op amps.

• Voltage rule for op amps.

• The inverting voltage amplifier.

• The non-inverting voltage amplifier.

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
amp comes reasonably close to achieving the ideal. An ideal amplifier should, in
addition to other desirable parameters, have at least an infinitely high input
impedance, an output impedance of zero ohms, an infinitely high gain and an
infinitely wide bandwidth. Table 1 compares some imprtant parameters such as
Input Impedance(ZIN), Input Bias Current (IIN),Large
Signal Voltage Gain (AV) and Output Impedance (ZOUT) of
some typical real (practical) op amps with the ‘ideal’ op-amp model:

Table 1: The Ideal Amplifier v The Practical
Op Amp

ZIN

IIN

AV

ZOUT

The Ideal Op-Amp

Infinity.

Zero.

Infinity

0Ω

741

2MΩ

80nA

316 to 200,000 (50dB to 106dB)

Depends on gain and feedback but typically less than 100Ω
to more than 1KΩ

TLC271

1TΩ

60pA

5,000 to 46,000 (74dB to 93dB) min.

LMC660

>1TΩ

0.002pA

40,000 to 990,000 (92dB to 126dB)

Negative Feedback

There are two basic methods of connection for op amp voltage amplifiers,
making the op amp into an inverting or a non-inverting voltage amplifier. In
each case, the voltage gain of the amplifier is set simply by the ratio of two
resistors. Using a very high gain amplifier and applying negative feedback
produces a very stable amplifier with a gain
that is is virtually independent of changes in temperature or variations in
semiconductor characteristics. Just as in the discrete component amplifiers
described in Amplifiers
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
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
identical. If any input signal or voltage tries to make the input potentials
different, the op amp output will change in the opposite polarity to the input
and, via the feedback loop, keep the difference between the two inputs at
0V.

Rule 2. The Current Rule.

Because the input impedance is infinitely high, no current can flow into
either input.

These rules used in relation to the ideal op amp can be used to make the
operation of the two main voltage amplifier circuits easier to understand. The
small differences between ideal and practical op amps can be temporarily
ignored.

Fig.6.7.3 The Inverting Amplifier

The Inverting Amplifier

Fig. 6.7.3 shows the inverting amplifier, in this configuration the signal
input is applied to the inverting (−) input to produce an anti-phase output
signal whose amplitude is Vin x Avcl where Avcl
is the closed loop gain of the op amp.

Negative feedback is used to reduce the op amp’s very high maximum gain to
the required level. The closed loop gain (Avcl) is set simply by the
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
assumed that:

Pin 3 is at 0V, as it is connected to ground via R3, which will have no
voltage developed across it as no current flows into pin 3 (Rule 2), so pin 2
will also be at 0V (Rule 1).

Therefore Rin and Rf are effectively connected in
series between Vin and Vout with pin 2 between the two
resistors held at 0V.

No current can flow into pin 2 (Rule 2) so the current flowing into
Vin cannot be flowing into the op amp, therefore it must be flowing
through Rf to the output.

The diagram in Fig. 6.7.3b shows that in effect Rin and
Rf are two resistors connected in series between Vin and
Vout with Pin 2 at 0V.

Although pin 2 is not actually connected to ground, it must be at the same
voltage as pin 3 (Rule 1) which is at 0V (because of Rule 2). This important
effect of making some location in a circuit that is not actually connected to
ground, yet maintains a potential of 0V is called ‘virtual earth’ (or virtual
ground).

Any input current (Iin) therefore flows directly from the input,
via Rin and Rf to the output, with Rin and
Rf forming a potential divider between the opposite polarity voltages
Vin and Vout with pin 2 at 0V.

The current through a series
resistor circuit is the same for each resistor, so the input voltage
Vin will be proportional to the resistance of Rin and the
voltage across Rf will be proportional to the resistance of
Rf

Notice that because Rf is
effectively connected between the output terminal and virtual earth (0V) the
voltage across Rf is also equal to Vout. This makes
Rf / Rin equal to Vout / Vin (the
gain of the amplifier) and therefore the closed loop gain of the inverting op
amp voltage amplifier (Avcl) is given by the equation:

Notice that the formula only tells you the ratio of the resistors and not
their actual values. Fortunately in audio amplifiers the values are not too
critical, usually keeping the resistor values between about 10K and 100K is OK.
However it is good to try and keep Rin as high a resistance as
possible, and also in a practical amplifier, rather that grounding the
non-inverting input directly it should be grounded by a resistor with the same
value as Rin to keep the (tiny) input currents equal. This gives a
better chance of the output voltage being zero volts (or close to it) when the
input is zero volts.

The Non-inverting Amplifier

Fig.6.7.4 The Non-Inverting Amplifier

In the non-inverting amplifier illustrated in Fig. 6.7.4a the input is
applied to the non-inverting input (pin 3) while the negative feedback resistor
(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
operation. As shown in Fig. 6.7.4b, the circuit does not have a virtual earth
point, but the bottom end of R1 is connected to ground, which means that pin 2
will follow the varying input voltage Vin on pin 3 (Rule 1).

Rf and R1 now form a potential divider between Vout and
0V. Just like the inverting amplifier, no current will flow into pin 2 (Rule 2)
so the voltages across R1 and Rf will be proportional to their
individual resistances. The ratio of these resistances, and therefore the ratio
of Vout to Vin i.e. the closed loop gain is given by the
standard potential
divider formula (Rf + R1) / R1.

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© 2007− 2016 Eric Coates MA BSc. (Hons) All
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