Chapter 6 BJT Amplifiers

Objectives Understand the concept of amplifiers Identify and apply internal transistor parameters Understand and analyze common-emitter, common-base, and common-collector amplifiers Discuss multistage amplifiers Troubleshoot amplifier circuits

Introduction One of the primary uses of a transistor is to amplify ac signals. This could be an audio signal or perhaps some high frequency radio signal. It has to be able to do this without distorting the original input.

Amplifier Operation Recall from the previous chapter that the purpose of dc biasing was to establish the Q-point for operation. The collector curves and load lines help us to relate the Q-point and its proximity to cutoff and saturation. The Q-point is best established where the signal variations do not cause the transistor to go into saturation or cutoff. What we are most interested in is the ac signal itself. Since the dc part of the overall signal is filtered out in most cases, we can view a transistor circuit in terms of just its ac component.

Amplifier Operation For the analysis of transistor circuits from both dc and ac perspectives, the ac subscripts are lower case and italicized. Instantaneous values use both italicized lower case letters and subscripts. Fig 6-1 V vs. t graph

Amplifier Operation The boundary between cutoff and saturation is called the linear region. A transistor which operates in the linear region is called a linear amplifier. Note that only the ac component reaches the load because of the capacitive coupling and that the output is 180º out of phase with input. Fig 6-2 amplifier circuit

Transistor Equivalent Circuits We can view transistor circuits by use of resistance or r parameters for better understanding. Since the base resistance, rb is small it normally is not considered and since the collector resistance, rc is fairly high we consider it as an open. The emitter resistance, rc is the main parameter that is viewed. You can determine rc from this simplified equation. rc = 25 mV/IE Fig 6-6 r-parameter comparison

Transistor Equivalent Circuits The two graphs best illustrate the difference between DC and ac. The two only differ slightly. Fig 6-7 IC vs IB curves

Transistor Equivalent Circuits Since r parameters are used throughout the rest of the textbook we will not go into deep discussion about h parameters. However, since some data sheets include or exclusively provide h parameters these formulas can be used to convert them to r parameters. r’e = hre/hoe r’c = hre + 1/hoe r’b = hie - (1+ hfe)

The Common-Emitter Amplifier The common-emitter amplifier exhibits high voltage and current gain. The output signal is 180º out of phase with the input. Now let’s use our dc and ac analysis methods to view this type of transistor circuit. Fig. 6-8 ce amp

The Common Emitter Amplifier DC Analysis The dc component of the circuit “sees” only the part of the circuit that is within the boundaries of C1, C2, and C3 as the dc will not pass through these components. The equivalent circuit for dc analysis is shown. The methods for dc analysis are just are the same as dealing with a voltage-divider circuit. Fig 6-9 dc eq. ce amp

Common Emitter Amplifier AC Equivalent Circuit The ac equivalent circuit basically replaces the capacitors with shorts, being that ac passes through easily through them. The power supplies are also effectively shorts to ground for ac analysis. Fig 6-10&6-11 ac eq. ce circuit

Common Emitter Amplifier AC Equivalent Circuit We can look at the input voltage in terms of the equivalent base circuit (ignore the other components from the previous diagram). Note the use of simple series-parallel analysis skills for determining Vin.

Common Emitter Amplifier AC Equivalent Circuit The input resistance as seen by the input voltage can be illustrated by the r parameter equivalent circuit. The simplified formula below is used. Rin(base) = acr’e The output resistance is for all practical purposes the value of RC. Fig 6-12 r parameter ce amp

Common Emitter Amplifier AC Equivalent Circuit Voltage gain can be easily determined by dividing the ac output voltage by the ac input voltage. Av = Vout/Vin = Vc/Vb Voltage gain can also be determined by the simplified formula below. Av = RC/r’e Fig 6-14

Common Emitter Amplifier AC Equivalent Circuit Taking the attenuation from the ac supply internal resistance and input resistance into consideration is included in the overall gain. A’v = (Vb/Vs)Av or A’v = Rin(total)/Rs + Rin(total) Fig 6-19

The Common-Emitter Amplifier The emitter bypass capacitor helps increase the gain by allowing the ac signal to pass more easily. The XC(bypass) should be about ten times less than RE. Fig 6-8 ce amp

The Common-Emitter Amplifier The bypass capacitor makes the gain unstable since transistor amplifier becomes more dependent on IE. This effect can be swamped or somewhat alleviated by adding another emitter resistor(RE1). Fig 6-18

The Common-Collector Amplifier The common-collector amplifier is usually referred to as the emitter follower because there is no phase inversion or voltage gain. The output is taken from the emitter. The common-collector amplifier’s main advantages are its high current gain and high input resistance. Fig 6-25 Emitter-follower

The Common-Collector Amplifier Because of its high input resistance the common-collector amplifier used as a buffer to reduce the loading effect of low impedance loads. The input resistance can be determined by the simplified formula below. Fig 6-26 emitter-follower equivalent Rin(base)  ac(r’e + Re)

The Common-Collector Amplifier The output resistance is very low. This makes it useful for driving low impedance loads. The current gain(Ai) is approximately ac. The voltage gain is approximately 1. The power gain is approximately equal to the current gain(Ai).

The Common-Collector Amplifier The darlington pair is used to boost the input impedance to reduce loading of high output impedance circuits. The collectors are joined together and the emitter of the input transistor is connected to the base of the output transistor. The input impedance can be determined the formula below. Rin = ac1ac2Re Fig 6-28 darlington pair

The Common-Base Amplifier The common-base amplifier has high voltage gain with a current gain no higher than 1. It has a low input resistance making it ideal for low impedance input sources. The ac signal is applied to the emitter and the output is taken from the collector. Fig 6-30

The Common-Base Amplifier The common-base voltage gain(Av) is approximately equal to Rc/r’e The current gain is approximately 1. The power gain is approximately equal to the voltage gain. The input resistance is approximately equal to r’e. The output resistance is approximately equal to RC.

Multistage Amplifiers Two or more amplifiers can be connected to increase the gain of an ac signal. The overall gain can be calculated by simply multiplying each gain together. A’v = Av1Av2Av3 …… Fig 6-32

Multistage Amplifiers Gain can be expressed in decibels(dB). The formula below can be used to express gain in decibels. A v(dB) = 20logAv Each stage’s gain can now can be simply added together for the total.

Multistage Amplifiers The capacitive coupling keeps dc bias voltages separate but allows the ac to pass through to the next stage. Fig 6-33 2 stage ce amp

Multistage Amplifiers The output of stage 1 is loaded by input of stage 2. This lowers the gain of stage 1. This ac equivalent circuit helps give a better understanding how loading can effect gain.

Multistage Amplifiers Direct coupling between stage improves low frequency gain. The disadvantage is that small changes in dc bias from temperature changes or supply variations becomes more pronounced. Fig 6-35 Direct-coupled amp

Troubleshooting Troubleshooting techniques for transistor amplifiers is similar to techniques covered in Chapter 2. Usage of knowledge of how an amplifier works, symptoms, and signal tracing are all valuable parts of troubleshooting. Needless to say experience is an excellent teacher but having a clear understanding of how these circuits work makes the troubleshooting process more efficient and understandable.

Troubleshooting The following slide is a diagram for a two stage common-emitter amplifier with correct voltages at various points. Utilize your knowledge of transistor amplifiers and troubleshooting techniques and imagine what the effects would be with various faulty components—for example, open resistors, shorted transistor junctions or capacitors. More importantly, how would the output be affected by these faults? In troubleshooting it is most important to understand the operation of a circuit. What faults could cause low or no output? What faults could cause a distorted output signal? Fig 6-36 2 stage ce amp

Troubleshooting Fig 6-36 2 stage ce amp

Summary Most transistors amplifiers are designed to operate in the linear region. Transistor circuits can be view in terms of its ac equivalent for better understanding. The common-emitter amplifier has high voltage and current gain. The common-collector has a high current gain and voltage gain of 1. It has a high input impedance and low output impedance.

Summary The common-base has a high voltage gain and a current gain of 1. It has a low input impedance and high output impedance Multistage amplifiers are amplifier circuits cascaded to increased gain. We can express gain in decibels (dB). Troubleshooting techniques used for individual transistor circuits can be applied to multistage amplifiers as well.