Chapter 5: BJT Amplifiers Electronic Devices Amplifier Operation DC Quantities use upper case roman subscripts. Example: V CE (The second letter in the subscript indicates the reference point). AC Quantities and time varying signals use lower case subscripts. Example: V ce. Instantaneous quantities are represented by both lowercase letters and subscripts such as i c, i e, i b, and v ce. Internal transistor resistances are indicated as lower case quantities with a prime and an appropriate subscript. An example is the internal ac emitter resistance, r e. External resistances are indicated as capital R with either a capital or lower case subscript depending on if it is a dc or ac resistance. Examples: R C and R c. The Figure 1 shows an example of a specific waveform for the collector emitter voltage. Figure 1 Linear Amplifier A linear amplifier provides amplification of a signal without any distortion so that the output signal is an exact amplified replica of the input signal. A voltage-divider biased transistor with a sinusoidal ac source capacitively coupled to the base through C 1 and a load capacitively coupled to the collector through C 2 is shown in Figure 2. Figure 2 37 Assist. Prof. Dr. Hamad Rahman
For the amplifier shown, notice that the voltage waveform is inverted between the input and output but has the same shape. Transistor Ac Models To visualize the operation of a transistor in an amplifier circuit, it is often useful to represent the device by a model circuit. A transistor model circuit uses various internal transistor parameters to represent its operation. Transistor models are described in this section based on resistance or r parameters. Another system of parameters, called h parameters. The five r parameters commonly used for BJTs are given in the following Table 1. The lowercase letter r with a prime denotes resistances internal to the transistor. r parameter Description α ac ac alpha (I c /I e ) β ac ac beta (I c /I b ) r e ac emitter resistance r b ac base resistance r c ac collector resistance The interpretation of this model circuit in terms of a transistor s ac operation is as follows: A resistance (r e) appears between the emitter and base terminals. This is the resistance seen looking into the emitter of a forward-biased transistor. The collector effectively acts as a dependent current source of α ac I e or, equivalently, β ac I b, represented by the diamond-shaped symbol, these factors are shown in Figure 3. Figure 3: Relation of transistor symbol to r-parameter model. It is also temperature dependent and is based on an ambient temperature of 20 C. r e 24mV I E H.W: Determine the r e of a transistor that is operating with a dc emitter current of 2mA. H.W: What is I E if r e = 8Ω? 38 Assist. Prof. Dr. Hamad Rahman
The Common-Emitter Amplifier Three amplifier configurations are the common-emitter, the common-base, and the common-collector. In the common-emitter (CE) amplifier, the input signal is applied to the base and the inverted output is taken from the collector. The emitter is common to ac signals. Figure 4 shows a common-emitter amplifier with voltage-divider bias and coupling capacitors C 1 and C 3 on the input and output and a bypass capacitor, C 2, from emitter to ground. The input signal, V in, is capacitively coupled to the base terminal, the output signal, V out, is capacitively coupled from the collector to the load. A CE amplifier has high voltage, current, and power gains, but a relatively low input resistance. Figure 4: A common-emitter amplifier. The Common-Collector Amplifier The common-collector (CC) amplifier is known as an emitter-follower (EF). In the common-collector (CC) amplifier, the input signal is applied to the base and the output is taken from the emitter. The collector is common to ac signals. A common-collector amplifier has high input resistance and high current gain, but its voltage gain is approximately 1. Figure 5: A common-collector amplifier. 39 Assist. Prof. Dr. Hamad Rahman
The Common-Base Amplifier The common-base (CB) amplifier provides high voltage gain with a maximum current gain of 1. Since it has a low input resistance, the CB amplifier is the most appropriate type for certain applications where sources tend to have very low-resistance outputs. Figure 6: Common-base amplifier with voltage-divider bias. Capacitors in Amplifier Coupling capacitors are used to transmit an ac signal from one node to another. Coupling capacitors provide dc isolation between two nodes. Bypass capacitor is used to short circuit ac signals to ground (while not affecting the dc operation of the circuit). The value of the bypass capacitor must be large enough so that its reactance over the frequency range of the amplifier is very small (ideally) compared to R E. The capacitive reactance, X C, of the bypass capacitor should be at least 10 times smaller than R E at the minimum frequency for which the amplifier must operate (10XC RE). Figure 7 Transistors as a Small Signal Amplifier There are 2 analysis; DC Analysis and AC Analysis. The purpose of DC analysis is to determine the initial operating values of I C, I B and V CE (Q-point). The goal is to set the Q-point such that it does not go into saturation or cutoff when an ac signal is applied. If 40 Assist. Prof. Dr. Hamad Rahman
the Q-point is in active region, the transistor can operate as an amplifier. The purpose of AC analysis is to obtain the gain. An amplifier is a system that has a gaining ability to amplify where a small electrical signal will be converted into a strong one. Amplifiers are classified as small signal amplifiers (preamplifiers) and strong signal amplifiers (power amplifiers). Amplifiers are able to amplify current, voltage and/or power. In other words, only amplifiers are able to produce power gain where as other devices such as transformer are only able to produce voltage and current gain. Small signal amplification causes small current changes and small output voltage change surrounding operation point (Q-point from DC analysis). These small changes are small enough for us to disregard any influence it may have on the transistor s parameter values such as α and β. There are 4 basic categories of small signal amplifiers: Voltage amplifier. Current Amplifier Trans-conductance Amplifier (converts voltage to current) Trans-resistance Amplifier (converts current to voltage) Gain The gain of an amplifier is the ratio of an output parameter to an input parameter. An amplifier with a current gain of 100 during normal use, the output current is a hundred times greater that the input current. There are three types of gain: Current gain Voltage gain Power gain A i = i o i i A v = v o v i A p = P o P i = A v A i 41 Assist. Prof. Dr. Hamad Rahman