Transistor characteristics

There are various test circuits that are used to measure the way in which a transistor behaves. The results of one of the more useful of such tests is illustrated opposite. The transistor is set up in the common emitter connection as on page 98 with additions to allow the base current and the collector emitter p.d. to be controlled and with meters to measure the base and collector currents. First look at the lowest curve in the figure. The base current I_B is set at 50 μA and the collector-emitter p.d. is gradually increased from 0 V to 30 V. At each stage, we measure the collector current I_c. Collector current begins at zero but rapidly increases to 10 mA. From then on, further increase in the collector-emitter p.d. produces virtually no further increase in I_C. The line is almost horizontal.

If we repeat the trial but make I_B equal to 100 μA (double the previous I_B), the shape of the curve is as before but now levels out with I_c equal to 20 mA (double the previous I_c). The same applies to the other two trials illustrated in the figure. In each test I_c levels out at a value that is 200 times that of I_B. Collector current is proportional to base current, with a current gain of 200.

The figure clearly demonstrates the amplifying action of the transistor. It also shows that, once the collector-emitter p.d. is greater than about 2 V, the amplifying action is independent of the actual collector-emitter p.d. Increases in I_B result in proportionate increases in I_C, but there is a limit to this, not shown in the figure. Once I_B has been increased above a certain value, there is no corresponding increase in I_C. Then we say that the transistor is saturated.

For a transistor to amplify as shown in the figure, it must be in its operating region. The lower limit of this region is when the collector-emitter p.d. is very low and the transistor is operating on the steeply sloping part of the curve. The upper limit of this region is when I_B is sufficient to saturate the transistor.

Common emitter amplifier

On the left is a single-transistor amplifier, using the transistor in the common emitter connection. The power supply voltage must be large enough to produce a suitable collector-emitter p.d. and thus provide one of the conditions mentioned above for putting the transistor into its operating region. In a typical radio receiver or tape player the supply voltage is between 3 V and 9 V, although some receivers operate on a supply of only 1.5 V.

The next consideration is the quiescent state of the amplifier. This is the state of the amplifier when it is operating but when it is not receiving any signal to amplify. In the quiescent state there is a constant base current and a constant (and larger) collector current. The collector is at a constant output voltage, measured with reference to the 0 V line. It is usual for the quiescent collector voltage to be half the supply voltage. When a signal is being amplified, the collector voltage rises and falls. If it is normally half-way between 0 V and the supply, it is able to rise and fall freely and equally in both directions. This makes it possible to have a large output signal without distortion. Usually, except in power amplifiers, the quiescent collector current is a few milliamps. Given this current, the value of R3 is selected to drop the voltage at the collector to the half-way level.

The transistor is biased into its operating region by the two biasing resistors R1 and R2. These act as a potential divider which provides sufficient base current to produce the required quiescent collector current. It is possible to bias a transistor with a single resistor either connected as Rl, or between the collector and base, but biasing with two resistors gives the amplifier greater stability.

The signal is fed into the amplifier through the coupling capacitor Cl (p. 46). An alternating signal arrives at the input and passes across the capacitor. It causes a small and varying current to be added to or subtracted from the constant base current supplied by R1/R2. In some circumstances, it is possible to omit the capacitor and connect the amplifier to the signal source, but capacitor coupling is more often employed. For example, as we shall see in the two-stage amplifier, the quiescent output voltage (half the supply voltage) of the first stage would bias the transistor of the second stage into saturation if it were directly connected. The coupling transistor allows there to be a constant p.d. across the transistor without affecting the transmission of alternating signals across it.

Input Impedance

The potential at the junction of the two resistors of a potential divider depends on their ratio. For example, it might be found that a suitable base current is provided by making R1 equal to 220 kΩ and R2 equal to 18 kΩ. If the supply voltage is 9 V, the potential at the base is 9 × 18 000/238 000 = 0.68 V. But the same potential could also be obtained if the resistors were 22 kΩ and 1.8 k, or even as small as 220 Ω and 18 Ω. In each case, the ratio of the resistances is the same.

Using smaller resistors would not affect the biasing of the transistor but it would have a serious effect on the input signal. Low-value resistors would short-circuit most of the tiny signal current to the 0 V or + V rails. Little of it would pass to the base. Most of the signal would be lost.

If the input resistance of the amplifier is too low, the signal is mostly lost. The same idea applies when we consider the fact that the capacitor, together with R1 and R2, make up a high-pass filter (p. 65). With certain values of the capacitor and resistors, the low-frequency portion of the signal is blocked and never reaches the transistor or, at least is appreciably reduced in amplitude. The combined resistive and capacitative effects on the input side are the input impedance. For the maximum signal to reach the transistor and be amplified, the input impedance must be as high as is feasible. This is achieved by making R1 and R2 as high as possible (if they are too high, the base current will not be large enough) and to select a value for the capacitor so that all required low frequencies are passed.

Output impedance

This is the impedance offered to the flow of current from the output side to the next stage of amplification or to a loudspeaker. In general, this should be as low as possible. The value of R3 should not be high, otherwise it may be impossible for enough current to flow from the supply line through R3 and on to the next stage. Similarly, if there is a coupling capacitor on the output side, the high-pass filter that it makes with R3 must pass all required low frequencies.

Input and output impedances vary with frequency. When specifying the performance of an amplifier, they are usually quoted for a signal frequency of 1 kHz.

Impedance matching

As explained above, if a circuit has low input impedance, a signal fed to it from another circuit is likely to be partially or almost wholly lost. Similarly, a circuit with high output impedance will not produce sufficient current to drive a subsequent stage of low input impedance. It can be shown that, for maximum transfer of power between two stages of a circuit, the output impedance of the first stage must equal the input impedance of the second stage.

Impedance matching is essential if power is not to be lost. One way of achieving this is to select suitable values of resistors and other components at the output and input. Most instances in which impedance matching is required involve feeding a low-impedance input from a high-impedance output. This can be done by linking the two circuits by an emitter follower amplifier (p. 99).

Two-stage amplifier

The amplifier below has two stages, connected one after the other in cascade. It does not simply amplify a signal and then amplify it still further. The function of the first stage, based on Ql, is to amplify the voltage of the signal produced by the microphone. The function of the second stage, based on Q2 is to amplify the current, making it sufficient to drive the loudspeaker.

The input from the crystal microphone is connected directly to the base of Ql. A crystal microphone has a high resistance so connecting it in parallel with R2 does not have any appreciable effect on the potential-divider action of Rl and R2. It does not upset the biasing of Ql so there is no need for a coupling capacitor. Ql is a high-gain transistor to amplify the input voltage from the microphone.

With the collector of Ql quiescent at around V/2, it is obvious that a capacitor C2 must be used to couple this stage to the next. The next stage has only a single bias resistor R4. R5 and R6 are chosen to bring the quiescent potential at the emitter of Q2 to the half-way point. Their values are relatively low to enable them to pass relatively high currents. Q2 is a low-gain power transistor able to pass a substantial current without over-heating.

The low values of R5 and R6 give the second stage a low output impedance, which means it can deliver a large current to the final stage, the loudspeaker.

With the collector of Ql quiescent at around V/2, it is obvious that a capacitor C2 must be used to couple this stage to the next. The next stage has only a single bias resistor R4. R5 and R6 are chosen to bring the quiescent potential at the emitter of Q2 to the half-way point. Their values are relatively low to enable them to pass relatively high currents. Q2 is a low-gain power transistor able to pass a substantial current without over-heating.

The low values of R5 and R6 give the second stage a low output impedance, which means it can deliver a large current to the final stage, the loudspeaker. Loudspeakers typically have very low impedance, sometimes as low as 4 Ω and rarely higher than 100 Ω, so they need a large current to drive them. As the current through R6 fluctuates, a varying p.d. is developed across it. This produces a signal at the junction of R6 and the emitter and hence at C3. The signal passes across C3 and drives the loudspeaker.

The action of Cl is as follows. When the potential at the base of Ql rises, base current increases and so does the collector current. The increased collector current passes through Ql and VRl. Increased current through VRl generates an increased p.d. between its ends. Its ‘lower’ end is fixed at 0 V, so the potential at its ‘upper’ end rises.

The factor that decides how much base current flows to Ql is the base-emitter p.d. We have just explained that a rise in base potential results in a rise in emitter potential. As the base current rises and the base-emitter p.d. is increased, the emitter potential rises too and the base-emitter p.d. is reduced. The actions are in opposite directions. We call this kind of action negative feedback. It could happen that the rises in base potential and emitter potential were equal. In such an event the base-emitter p.d. remains constant, so the base current remains constant and likewise the collector current. The signal is completely damped out. No sound is heard.

The function of Cl is to absorb part of the change in base-emitter p.d., so that the amount of feedback can be controlled. The capacitor taps off part of VRl. Fluctuations in the p.d. across the tapped-off part are damped out by the action of the capacitor. Only the fluctuations in the part of VRl between the emitter and the tapping are available as feedback.

By allowing a certain amount of feedback, the gain of the amplifier is reduced. This seems to be a fault, but it is compensated for by the fact that the fidelity of the amplifier is much improved. Its gain is constant over the whole of the frequency range and it is not affected by signal amplitude. In other words, it has a level response. In addition, the gain is not affected by temperature, as it is when there is no feedback.

FET amplifier

As explained on page 85, an FET has high input impedance. This makes it very suitable for use as the first stage of an amplifier that is to receive its input from a very high impedance source, such as a capacitor microphone (p. 119). In the circuit below, the current through R2 causes a p.d. to develop across R2 when it is in the quiescent state. For reasons given earlier, the value of R2 is chosen so that the p.d. is about half the supply voltage. In other words, the source terminal of Q1 is at a positive potential. The gate is held at zero potential by Rl. Consequently, the gate potential is negative with respect to the source potential, as required for the operation of the transistor.

The signal passes through Cl to the gate, making the gate potential fluctuate and resulting in a varying current through the transistor. Note that although the gate of Q1 offers high input impedance, the impedance of the amplifier is the impedance between the input terminal and the 0 V rail. This impedance is the resistance of Rl, typically 1 MΩ, which is high enough for most applications.

The transistor and R2 both have low resistance (a few hundred ohms) so the output of this amplifier has low impedance. Capacitor C2 couples this amplifier to later stages which may be based either on bipolar or field effect transistors.

This circuit is a common-drain or source-follower amplifier. It does not produce any voltage gain but is invaluable for connecting a high-impedance source with a low-impedance amplifier. High-impedance capacitance microphones of the electret type frequently have an FET amplifier built in to them, powered by a small dry cell. This matches the very high impedance of these microphones to the medium-impedance amplifier or tape recorder to which they may be connected. When such microphones are used at the ends of long screened cables, the high-impedance of the microphone acts in combination with the capacitance of the screened cable to produce, in effect, a low-pass filter. The filter reduces the treble component of the signal, giving a muffled sound after amplification. This type of distortion is reduced by the use of the FET pre-amplifier in the microphone case, for its medium-impedance output is not affected by cable capacitance to the same extent.

MOSFET amplifiers

A typical MOSFET common source amplifier is shown below. The transistor is an n-channel MOSFET so it operates with its gate at a positive potential. The bias is obtained from a potential divider as in the BJT amplifier on page 183. The gate requires virtually no current to bias it. R1 and R2 may each be several hundred of kilohms and a third resistor R3 may be placed between the potential divider R1/R2 and the gate. R3 may have a resistance as high as 10 MΩ, so the input impedance of the amplifier is in excess of 10 MΩ, which is very high indeed.

Low-power MOSFETs have a rapid response time, making MOSFET amplifiers useful in radio-frequency circuits. Amplifiers based on power MOSFETs (such as VMOS and HEXFETs, p. 82) are recommended for power-control circuits. They have greater thermal stability than bipolar transistors and are not subject to thermal runaway (p. 70). They may be wired in parallel to enable very large currents to be controlled, in fact a HEXFET really consists of numerous paralleled MOSFETs.

Low-noise amplifiers

In the electronic sense, noise is an unwanted signal that is superimposed on a wanted signal. Much of a designer’s effort goes toward reducing it.

Electronic noise may become apparent as actual noise, in the more usual sense, when we hear a crackling, humming, or hissing sounds in the background when listening to the radio. Crackling may be caused by electrical equipment in the neighbourhood switching on and off. Refrigerators are a common source of such noise. The spikes on the mains supply travel along the mains wiring and enter the power supply of the radio set. One way to reduce such interference is to pass the power supply leads through ferrite beads (p. 53).

Lightning is a powerful source of electromagnetic interference (EMI) in radio transmissions and little can be done to eliminate it. Any sort of magnetic field can be picked up by conductors in a circuit and, if it is picked up in a suitable place, can be amplified. Connections such as microphone cables are made with screened leads to prevent such EMI overwhelming the small signals from the microphone. The screen consists of a sheath of metallic braid, connected to the earthed (0 V) line of the amplifier. Tuning coils (p. 55) are usually cased in a metal shield, which serves to prevent the coil from either picking up EMI from close-by circuitry, or radiating it to other parts of the circuit. In today’s congested conditions the subject of EMI is an important one and much legislation exists to limit emissions of EMI.

Precautions can usually reduce or eliminate noise coming from outside a circuit, but there are other sources of noise that arise within the circuit itself. An electric current is a stream of electrons or holes. There is a random element to their movements, giving rise to small random fluctuations in voltage. When amplified, this can produce a hissing sound, superimposed on the normal signal. This is often known as white noise. This type of noise is produced whenever current flows through any kind of resistance. To minimize it, we must try to reduce the resistance of circuits, but this is not always practicable. The random movements are greater at higher temperatures, so keeping an amplifier cool will help reduce noise. Another source of noise is that even with a steady current the actual number of particles moving past a given point in a given time is not absolutely constant. There are random fluctuations in current and these too lead to noise.

In non-audio circuits, noise can show itself in appropriate ways, for example as ‘snow’ on a TV screen. In a control system, it may be manifest as erratic responses. In a security system, it may lead to false alarms. In a digital system, noise may result in a digital ‘0’ being read as a ‘I’, and the other way about, with possibly disastrous results.

Noise can be added to a system at any stage but the biggest problems arise when it is added at an early stage. If the early stage of an amplifier is noisy, the noise is amplified along with the signal and may be very difficult to eliminate.

It is therefore very important to concentrate on the early stages of amplification. Low-noise transistors, reduction of resistance as much as possible, and filtering of the signal to remove all except the desired signal are all measures that can be taken to reduce noise. FETs produce less noise than BJTs and so are often used in the early stages of amplification, then may be followed by BJTs.

A good example of the need for low-noise amplifiers is illustrated in the title photograph of this chapter. This is one of the smaller radio-telescopes at Jodrell Bank. The signal reaching the telescope from Space is extremely weak and all possible noise-avoidance techniques are used. The antenna and the first stages of amplification are located at the focal point of the reflector, mounted on a structure of four girders. The signal is amplified immediately it is received, before it can be contaminated with signals from terrestrial sources. Various amplifiers are used at the focus, all mounted on a carousel inside casing. Amplifiers can be brought to the focal point one at a time on a rotating head. These are narrow-band amplifiers, tuned to operate at a single frequency. This is a good way of limiting noise since some types of noise are proportional to band-width. A narrow-band amplifier is much less susceptible to noise than a wide-band amplifier. The amplifier itself may be a special type known as a parametric amplifier, which has low noise properties. Another noise-reduction technique employed at Jodrell Bank is to cool the receiver and amplifier to 14 K (-259 °C) with liquid helium. Low temperature reduces the random vibrations of the electrons, so reducing a significant noise source.