Specification of Transistor Frequency Response

Basic BJT Amplifier Configurations

There are plenty of texts around on basic electronics, so this is a very brief look at the three basic ways in which a bipolar junction transistor (BJT) can be used. In each case, one terminal is common to both the input and output signal. All the circuits shown here are without bias circuits and power supplies for clarity.

Common Emitter Configuration

Here the emitter terminal is common to both the input and output signal. The arrangement is the same for a PNP transistor. Used in this way the transistor has the advantages of a medium input impedance, medium output impedance, high voltage gain and high current gain.

Common Base Configuration

Here the base is the common terminal. Used frequently for RF applications, this stage has the following properties. Low input impedance, high output impedance, unity (or less) current gain and high voltage gain.

Common Collector Configuration

This last configuration is also more commonly known as the emitter follower. This is because the input signal applied at the base is "followed" quite closely at the emitter with a voltage gain close to unity. The properties are a high input impedance, a very low output impedance, a unity (or less) voltage gain and a high current gain. This circuit is also used extensively as a "buffer" converting impedances or for feeding or driving long cables or low impedance loads.

Frequency response

Frequency response is the measure of any system's output spectrum in response to an input signal. In the audible range it is usually referred to in connection with electronic amplifiers, microphones and loudspeakers. Radio spectrum frequency response can refer to measurements of coaxial cables, category cables, video switchers and wireless communications devices. Subsonic frequency response measurements can include earthquakes and electroencephalography (brain waves).

Frequency response requirements differ depending on the application. In high fidelity audio, an amplifier requires a frequency response of at least 20–20,000 Hz, with a tolerance as tight as ±0.1 dB in the mid-range frequencies around 1000 Hz, however, in telephony, a frequency response of 400–4,000 Hz, with a tolerance of ±1 dB is sufficient for intelligibility of speech.

Frequency response curves are often used to indicate the accuracy of electronic components or systems. When a system or component reproduces all desired input signals with no emphasis or attenuation of a particular frequency band, the system or component is said to be "flat", or to have a flat frequency response curve.

The frequency response is typically characterized by the magnitude of the system's response, measured in decibels (dB), and the phase, measured in radians, versus frequency. The frequency response of a system can be measured by applying a test signal, for example:

applying an impulse to the system and measuring its response (see impulse response)

sweeping a constant-amplitude pure tone through the bandwidth of interest and measuring the output level and phase shift relative to the input

applying a signal with a wide frequency spectrum (for example digitally-generated maximum length sequence noise, or analog filtered white noise equivalent, like pink noise), and calculating the impulse response by deconvolution of this input signal and the output signal of the system.

These typical response measurements can be plotted in two ways: by plotting the magnitude and phase measurements to obtain a Bode plot or by plotting the imaginary part of the frequency response against the real part of the frequency response to obtain a Nyquist plot.

Once a frequency response has been measured (e.g., as an impulse response), providing the system is linear and time-invariant, its characteristic can be approximated with arbitrary accuracy by a digital filter. Similarly, if a system is demonstrated to have a poor frequency response, a digital or analog filter can be applied to the signals prior to their reproduction to compensate for these deficiencies.

Frequency response measurements can be used directly to quantify system performance and design control systems. However, frequency response analysis is not suggested if the system has slow dynamics

Amplifier Frequency Response

The essential purpose of an amplifier is to accept an input signal and provide an enhanced copy of that

signal as an output. However there is a fundamental relationship between signal frequency and gain such

that a given gain cannot be maintained over an arbitrarily large frequency range. Physically it takes time for

electric charge in a device to redistribute itself in response to a control signal, and so the response of a

device to a control signal inevitably becomes jumbled for very fast signal changes. This is an ultimate limit

to circuit response; degradation of the response may begin at lower signal frequencies because of delays

associated with other circuit components. Circuit components can introduce degradation of the frequency

response of a circuit at low frequencies as well as high, as will be seen.

In general the frequency response of an electronic circuit, e.g., the

transfer gain of the circuit, has the general appearance illustrated.

There is a ‘mid-band’ range of operation for which the gain is

substantially independent of frequency, bounded by ‘high’ and

‘low’ frequency ranges in which the gain is degraded. An

amplifier is ‘wide-band’ if the ratio of a frequency measuring the

onset of the high-frequency degradation to a corresponding

frequency for low frequency degradation is relatively large. A basic audio amplifier, for example, has a

substantially ‘flat’ response extending from about 100 Hz to roughly 10KHz. ‘Narrow-band’ amplifiers,

used for more specialized purposes, approximate selective amplification at a single frequency. Our basic

interest here is in wide-band amplifiers.

To simplify consideration of the frequency response of wide-band amplifiers analysis generally is

separated into three frequency ranges. The argument used to justify this separation is that those circuit

components associated with low-frequency degradation have by definition lost significant influence on the

response in mid-band, and supposing a monotonic behavior have no influence on the high-frequency

response. The converse argument removes the influence at low frequencies of those components affecting

the high frequency response. And, of course, in mid-band by definition neither of these sets of

components influences the response significantly. The mid-band range is the one assumed in previous

work, for example by neglecting the influence of coupling and bypass capacitors.

Initially we assume that frequency constraints are associated with circuit components o the r than the active

devices. Taking intrinsic device limitations into account is done later as an extension of the basic

procedures.

High Frequency Compensation:

Video Amplifiers

A video amplifier is used to amplify video from TVs, cameras, computer graphic devices, etc. Aside from having sufficient bandwidth and the ability to drive long cables: they cannot invert the signal's polarity; if they did: unless you were using an even number of amplifiers in cascade, the image would end up a negative. If you wanted a gain stage, but didn't want the signal to be inverted, you would drive the emitter instead of the base. This works, but as you might imagine, the input impedance is quite low. So by using what we learned about emitter followers back in chapter 219, we can "transform impedances," and now the noninverting video amplifier looks better.

Bandwidth of an Amplifier

Most amplifiers have relatively constant gain over a certain range (band) of frequencies, this is called the bandwidth (BW) of the amplifier.

As the frequency response curve shows, the gain of an amplifier remains relatively constant across a band of frequencies.

When the operating frequency starts to go outside this frequency range, the gain begins to drop off.

Two frequencies of interest, fC1 and fC2, are identified as the lower and upper cutoff frequencies.

The Bandwidth is found as: BW = fC2 – fC1

The operating frequency of an amplifier is equal to the geometric center frequency fo,

fo = √(fC1 fC2 )

Notice that the ration of fo to fC1 equals the ratio of fC2 to fo , this is:

fo / fC1 = fC2 / fo

Therefore we also have that:

fC1 = fo2 / fC2 ; fC2 = fo2 / fC1