Figure 1. Block diagram of system incorporating power amplification.

Transcription

1 It is often necessary use a circuit which has very low power capabilities to drive a system which has relatively high power requirements. This is typically accomplished by using an amplifier as an intermediate stage between the two systems, as shown in Figure 1. For example, in a phonographic audio system, the signal produced by the deflection of the phonograph needle has very little power this signal must be amplified before it can be sent to the speakers and produce audible sound. Figure 1. Block diagram of system incorporating power amplification. Another example of amplification is the dusk-to-dawn light from Lab 1. In that light, a bipolar junction transistor (BJT) was used as a switch to turn the light on or off. When the base voltage of the BJT was low, the BJT conducted no current and the light was off. When the BJT base voltage went high, the BJT conducted current and the light turns on. This system is a very simple amplifier a small current into the base is increased by some multiplicative factor and delivered to the light emitting diode. The amplifier in our dusk-to-dawn light, however, could only provide a positive voltage to the load resulting from a positive input voltage. The amplifier cannot reverse the polarity of the load voltage, nor can we apply a negative voltage to the amplifier and have the amplifier function 1. In this document, we will see an amplifier which can provide both positive and negative voltages to a load, in response to positive and negative input voltages. The amplifier we will consider is called a class B amplifier 2. We choose this type of amplifier in this section because it is probably most similar to the amplifier in the dusk-to-dawn light assignment. A class B amplifier contains two bipolar junction transistors (BJTs), one npn transistor and one pnp transistor. These designations describe the physical structure of the transistor. In this section, we will not be concerned with the details of the physics of npn and pnp transistors we 1 We CAN, of course, provide a negative input voltage. The amplifier, however, will not do anything as a result of a negative input voltage the output voltage will simply be zero. Try it and see for yourself! 2 There are various classes of amplifiers class A, class B, class AB, class C, these classes are based (in general) on the number of transistors in the amplifier and over what portion of the input signal the various transistors are providing current to the load. 1

2 will be simply concerned with the way in which these devices operate 3. Circuit symbols for npn and pnp BJTs are shown in Figure 1. In Figure 1, please note the direction of the arrow on the emitter on both BJTs. The direction of this arrow indicates the direction of the current through the emitter. Simplistic descriptions of the operation of npn and pnp BJTs are provided below: npn BJTs operate essentially as described previously: current flows out of the emitter of an npn BJT when a positive voltage is applied to its base, the current out of the emitter increases as the base voltage increases. The BJTs we used in Project 1 and the Key Concepts relative to Buffer Amplifiers associated with this project are both npn transistors. pnp BJTs operate, in some sense, in the opposite way as an npn BJT: current flows out of the emitter if a negative voltage is applied to the base of the BJT, the current out of the emitter increases as the base voltage decreases (becomes more negative). The references to emitter current above are based on the definition of emitter current direction indicated by the arrows in Figure 1. Since the npn and pnp BJTs are in some sense acting opposite to each other, the two types of BJTs are said to be complementary. Figure 1. Circuit symbols for BJT. A schematic of a class B amplifier is shown in Figure 2. The class B amplifier consists of an npn BJT, a pnp BJT, and two voltage supplies one positive relative to ground, and the other negative relative to ground. The low-power voltage to be amplified, V in, is applied to the base of both the npn and the pnp transistors. The collectors of the npn BJT and the pnp BJTs are connected to the positive and negative voltage supplies, respectively. The emitters of the two BJTs are both connected to the load circuit. 3 Transistors are created from n-type and p-type semiconductors. Crudely speaking, npn BJTs consist of a p-type semiconductor sandwiched between two n-type semiconductors. Likewise, pnp BJTs consist of an n-type semiconductor sandwiched between two p-type semiconductors. 2

3 Figure 2. Class B amplifier circuit. The amplifier shown in Figure 2 works as follows: If the voltage V in is positive relative to ground, the pnp transistor will be off and will have no current flow out of the emitter. The npn transistor, however, will allow current to flow from the positive voltage supply, into its collector, and out of its emitter. This current will then flow through the load circuit, as shown in Figure 3(a) below. As V in becomes more positive, the current through the load increases, and the voltage across the load increases. If the voltage V in is negative relative to ground, the npn transistor will be off and will have no current flow out of the emitter. The pnp transistor, however, will allow current to flow from ground, through the load circuit, and to the negative voltage supply. This operating condition is shown in Figure 3(b) below. Since current flows from higher to lower voltages, this operating condition will have the effect of reversing the polarity of the load voltage, as indicated in Figure 3(b). As V in becomes more negative, the current through the load becomes larger (in the direction shown in Figure 3(b)), and the voltage across the load becomes more negative. 3

4 (a) V in >0 (b) V in <0 Figure 3. Class B amplifier operation. There is one important aspect of BJT operation that we have ignored so far. The BJTs in Figures 2 and 3 do not turn on until a threshold voltage, V TH, is exceeded. For npn BJTs, the threshold is a relatively small positive voltage (often around 0.7V). For pnp BJTs, the threshold is a relatively small negative voltage (often around 0.7V). In reality, then, the npn BJT doesn t turn on until V in > V TH, and the pnp BJT doesn t turn on until V in < -V TH. Therefore, there is a range of input voltages for which neither BJT in Figure 2 is on (e.g. V TH <V in <V TH ), and for which there will be no voltage applied to the load. This is a generally undesirable characteristic of the amplifier of Figure 2 4 ; there are a number of more advanced design approaches that attempt to reduce this effect. 4 The lack of response of the amplifier to small values of Vin results in what it is called crossover distortion or Total Harmonic Distortion, or THD. This is an important characteristic for, say, audio amplifiers, since crossover distortion has an unpleasant effect on the sound produced by the amplifier. If you look at the specifications for an audio amplifier, you will likely find a number for THD. The effect above is what this specification is quantifying. We will introduce THD in more detail in later projects. 4

5 Notes: A class B amplifier can be used to amplify the power in signals which require both positive and negative polarities relative to ground. The class B amplifier requires two BJTs, one npn BJT and one pnp BJT. Since these BJTs act (in a sense) in opposite ways to one another, they are said to be complimentary. The two transistors are connected such that they cannot simultaneously conduct; the npn transistor conducts when the input voltage is positive, and the pnp transistor conducts when the input voltage is negative. The amplifier of Figure 2 is said to operate in a push-pull fashion. When the input voltage is positive, the npn BJT pushes current into the load. When the input voltage is negative, the pnp BJT pulls current from the load. The class B amplifier of Figure 2 is therefore sometimes referred to as a push-pull amplifier. Since the two BJTs in the class B amplifier are complementary, the amplifier of Figure 2 is sometimes called a complementary push-pull amplifier. Neither transistor in the amplifier of Figure 2 will conduct if the input voltage is in a range between their two threshold voltages. Therefore, the output voltage of the amplifier will be zero if V TH <V in <V TH. This results in a so-called dead band in the output for this range of inputs; this dead band is responsible for crossover distortion in the amplifier output. 5