Can a linear amplifier have finite bandwidth?

Question

I am reading Sedra and Smith and just want to be sure that the following characterization of an ideal linear amplifier is correct. A linear amplifier was defined as an amplifier for which the output signal is proportional to the input signal given any input signal. Supposing that we define bandwidth as the range of frequencies over which gain is constant; it then seems to me that one can say that an amplifier is linear if and only if it has infinite bandwidth (otherwise you could construct signals which superpose frequency components with different gains and get distortion through the amplifier). Is this correct?

Here is the definition from Sedra and Smith which I quote:

Answer 1

The text you quoted is not a definition of a linear amplifier. It says

Equation (1.4) is a linear relationship; hence the amplifier it describes is a linear amplifier.

Equation (1.4) is not a definition of a linear system or a linear amplifier, it is only one example of a linear relationship.

And an amplifier that follows Equation (1.4) is not the only kind of linear amplifier, it is only one example of a linear amplifier.

An amplifier that follows Equation (1.4) has infinite bandwidth, but other kinds of linear amplifiers do not.

The text continues,

it should be easy to see that if the relationship between \$v_o\$ and \$v_i\$ contains higher powers of \$v_i\$ then the waveform of \$v_o\$ will no longer be identical to that of \$v_i\$. The amplifier is then said to exhibit nonlinear distortion.

Here they do seem to treat the previous equation as definitional.

But you can obtain the same result (higher powers of \$v_i\$ are not allowed in the equation for \$v_o\$) if you use the proper definition of linearity, which has been given in Neil UK's answer.

On the other hand, derivative and integral terms (\$\frac{dv_i}{dt}\$ and \$\int v_i(t) dt\$) are allowed in linear relationships, and integral terms are the ones that result in limited bandwidth.

Answer 2

TL;DR As engineers, Sedra/Smith make one more assumption than the OP does. Engineers also make one more assumption than do mathematicians.

Mathematicians say a system H is linear iff (if and only if) it obeys the superposition principle, that H(x+y) = H(x) + H(y).

This is weaker than engineers tend to use, as it allows a time-varying system gain, which can generate output harmonics under certain conditions. So time-independent gain implies superposition, but superposition does not imply time-independent gain.

Engineers tend to be lazy and say linear when they mean LTI - Linear Time-Invariant.

An LTI system can still have a frequency response. Engineers tend to say that an amplifier is linear when it has a gain Vout/Vin = k(f), where k can vary with frequency. When excited by any single sinuosoid, the output will look like a perfect scaled copy of the input.

If k varies with frequency, then when excited by multiple sinusoids, the output will not look like the input, but there will be no intermodulation distortion, the only output frequencies will be those present in the input.

Now for the important difference in assumptions. If the input range of frequencies is limited to the flat frequency response part of the spectrum (a condition that is often assumed but not stated), and the amplifier also has linear phase - that is constant signal delay, then the output will look like a scaled version of the input, and finally we can write Vout/Vin = constant.

Plenty of amplifiers and filters exist where phase is not linear even though the gain may be, and the output of those does not look like the input. The output is usually referred to as suffering from phase distortion.

An ideal amplifier will have infinite bandwidth and zero noise, and not suffer clipping up to any arbitrary signal level.

The response of inductors or capacitors is linear, they obey superposition. Networks involving those do not obey Vout/Vin = k, there is a differential or integral operator involved.

Engineers do also use time-varying systems, although they call them mixers. A diode ring pumped with a strong local oscillator (LO) drive changes its gain with the LO period. When a low level signal is passed through, it does obey superposition, and engineers do talk about 'linear' mixers. A good mixer obeys superposition up to higher signal levels than a bad mixer. In a good mixer, the signal passing through does not suffer any intermodulation distortion between its frequency components. There is intermodulation between the LO and the signal components, that's how you get the sum and difference frequencies!

Answer 3

Real op amps (non-ideal) will have finite bandwidths. Perhaps this definition by Allan Hambley can enhance your understanding: -

"Ideal op amps have infinite gain magnitude and unlimited bandwidth. Real op amps have finite open-loop gain magnitude, typically between \$10^4 \$ and \$10^6 \$. Furthermore, the bandwidth of real op amps is limited. The gain of a real op amp is a function of frequency, becoming smaller in magnitude at higher frequencies."

Answer 4

Yes, you're right.

We're playing fast and loose with the idea of linearity, here, though. From the perspective of linearity being the "the straightness of the line of the graph of instantaneous input voltage versus instantaneous output voltage", you are exactly right. And, don't forget phase, another contributor to non-linearity in this sense.

If we are talking about linearity in the sense of the equations describing the relationship between input and output being linear, then that's a different story. Here we are referring to the idea that the transfer function obeys the conditions for algebraic linearity, such as commutativity and distributivity, like:

$$ f(x) \cdot g(x) = g(x) \cdot f(x)$$

$$ 2f(x) + 2g(x) = 2(f(x) + g(x)) $$

There is naturally a connection between these two interpretations, but I believe you are referring strictly to the former definition, in which case an ideal amplifier must indeed have infinite bandwidth, for the reasons you identified (and others, such as phase response and output slew rate).