EXPERIMENT 1: LOW AND HIGH FREQUENCY REGION ANALYSIS OF BJT AMPLIFIERS

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1 EXPERIMENT 1: LOW AND HIGH FREQUENCY REGION ANALYSIS OF BJT AMPLIFIERS Objective: In single layer common emitter amplifiers, observation of frequency dependence. Materials Transistor: 1x BC237 transistor Capacitor: 2x10μF, 1x470μF, 1x1NF, Resistance: 1x82kΩ, 1x8.2kΩ, 1x1kΩ, 1x3.3kΩ, 1x12kΩ 1. AMPLIFIER CIRCUITS Amplifier circuits known as conversion circuits, convert the small amplitude signals to large amplitude signals. Input signals of amplifier circuits increase by voltage gain at the output node. Output power will be amplified input power. (In transformers, the power obtained by the primary output is equal to the power obtained by the secondary output. VP IP = VS IS) Circuits which consist multiple transistors and formed by connecting the output of the previous amplifier to the input of another similar amplifier(cascade connection) are called multilayer amplifiers. Such amplifiers used for obtaining high gain. BJT amplifiers are used in three different connection types according to application areas and desired performance. 1.1 Common Emittor Amplifier Common emitter circuit is an amplifier type which uses emitter between input and output commonly. Common emitter circuit is used more widely than other amplifier types. The power gain obtained at the output is very high compared to other amplifiers. Also, since the input resistance of the circuit is high, it is suitable for the cascade connection. The basic circuit of the common emitter amplifier is shown in Figure 1.a. In Figure 1.b, the transition curve of the amplifier (input-output curve) is given and the amplification process is easily seen. Figure 1.a Figure 1.b 1

2 As can be seen from Figure 1.a, 180 degree phase difference is present between input and output. According to the values given on the transition curve in Figure 1.b, the amplifier circuit in Figure 1.a boosted the input signal Vi by 10 times. (1) If the voltage level of the input signal is too low or too high, the output signal will be distorted (clipped), this causes the obtaining distortion signal at the output. 1.2 Common Collector Amplifier Common collector circuit is an amplifier type which uses collector between input and output commonly. The circuit has high input resistance and low output resistance. But the obtained voltage and power gain is very low compared to the gain from common emitter amplifier. The voltage gain is close to Common Base Amplifier Common base circuit is an amplifier type which uses base between input and output commonly. Although the voltage gain obtained from this amplifier is high, the input resistance is low and the output resistance is high. The current gain is close to 1. Important values that determine the performance of the amplifier are as follows; Voltage Gain Current Gain Power Gain Input Resistance Output Resistance When designing the amplifier, component values of circuit and amplifier types are selected to obtain desired values. Furthermore, the voltage level of the input must be adjusted to produce minimum distortion (distortion) at the output. 2. THEORICAL ANALYSIS OF COMMON EMITTER AMPLIFIER In this experiment the theoretical analysis of the circuit in Fig. 2 should be done to observe the dependency of common emitter amplifier to instrument values and frequency. As seen from the amplifier circuit in Figure 2, the input voltage is equal to the base voltage and the output voltage is equal to the collector voltage. When the circuit is examined under DC conditions, the capacitors in the circuit must be considered as open circuited. And it is shortcircuited when it is examined under AC conditions (medium frequency range). With these conditions, if the components used in the common emitter circuit in Fig. 2 are examined; 2

3 C1 capacitor is used to isolate the incoming DC signal from the input. C2 capacitor prevents the DC signal from being sent to the circuit connected to the output of the amplifier. Capacitors which are used to isolate circuits from the DC components are called coupling capacitors. CE is short-circuited under AC conditions and removes the negative effect of the resistor RE on gain. Capacitors used for short-circuiting unwanted elements in terms of variable signals are called bridging capacitors. RE, emitter resistance, when examined under DC conditions, has positive effect on the circuit. RE resistance reduces the dependence of the amplifier's characteristics on the transistor parameters and brings about curative effects. Changes in the collector current and the β value of the transistor due to leakage currents in the transistor will not cause a large shift in the operating point due to RE resistance. The presence of non-bridged resistors on the emitter under AC conditions will result in an absolute reduction of the circuit gain. The resistors R1 and R2 are used as the voltage divider in the circuit so the values of IB current and therefore the IC current can be controlled. Figure 2 3

4 2.1 Common Emitter Amplifier in Low Frequency Region Coupling and bridging capacitors will have an effect on the alternating signal characteristics of the circuit if the frequency is reduced. These capacities are in the order of μf. At lower frequencies, the impedance values of the capacitors increase. Xc = 12πf. They will have a value that cannot be ignored. With the effect of the impedance values, gain obtained from amplifier will be reduced. Capacitors formed between the junctions due to the internal structure of the transistor act as an open circuit due to the capacitance values as mentioned before. Figure 3 shows the equivalent circuit model for the low frequency region of the common emitter amplifier. Figure Common Emitter Amplifier in Mid-Frequency Region When the amplifier is examined in the mid-frequency region, coupling and bridging capacitors C1, C2, CE are short-circuited. The transistors used in the amplifier have internal capacitances due to their structure. These capacities occurs between the junctions in the structure of the semiconductor elements. These capacities are in the order of pf. When the amplifiers are examined in medium and low frequency regions, the impedance values of these capacitors can be considered as open circuits because they are very large. In Figure 4, the equivalent circuit model is given for the middle frequency region of the common emitter amplifier. Figure 4 4

2.3 Common Emitter Amplifier in High Frequency Region When the frequency increases the effects of the capacitances between the junctions, which originate from the internal structure of the

5 2.3 Common Emitter Amplifier in High Frequency Region When the frequency increases the effects of the capacitances between the junctions, which originate from the internal structure of the transistor, will increase. At high frequencies, this causes a reduction in the gain of the amplifier. In Figure 5, the equivalent circuit model is given in the high frequency region of the common emitter amplifier. 2.4 Gain-Frequency Curve of Amplifier Figure 5 When the gain-frequency curve of the amplifier is plotted according to previous information, the curve shown in Fig. 6 will be obtained. The points at which the gain is reduced by 3 db as shown by the gain frequency curve are called corner frequencies. To obtain maximum gain from the amplifier, the amplifier must be operated in the mid-frequency region. fl is lower cut-off frequency and fh is upper cut-off frequency. The area between the corner frequencies is called the bandwidth of the amplifier (bandwidth, BW). BW = fh fl (2) Av(dB) = 20log Av (3) 5

6 3. EXPERIMENTAL PROCEDURE Figure Before the Experiment 1) Search BC237 for catalog information which should be used in the theoretical calculations to be made in the experiment and the report. 2) Examine the circuit in Figure 7 under DC operating conditions. (ICQ and VCEQ values at the DC operating point of the circuit must be calculated using the instrument values of the inspected device in the experiment). 3) Find the input / output impedances in the mid-frequency region of the same circuit and the voltage gain of the amplifier. 4) Find the frequency values of the lower and upper cut-off. 5) Show the frequency values obtained in the calculations on the gain-frequency curve. (Course notes for calculations will suffice. In the calculations, the necessary parameters for the transistors must be obtained from the catalogs.) 3.2 During the Experiment 1) Set up the circuit given in Fig. (Vin is a variable frequency sinusoidal voltage source.) 2) Apply f = 1Khz and Vin = 20mV (peak to peak). Find voltage gain with and without CE. 3) Change the frequency of the signal generator to determine the frequencies of the lower and upper cut-off. (Av (db) max - 3dB) points must be corresponded to Av / ) Figure 7 6

3.3 Experimental Results 1) If the voltage level of the input signal is low and high, the output signal is distorted. Show this on the input-output curve.

7 3.3 Experimental Results 1) If the voltage level of the input signal is low and high, the output signal is distorted. Show this on the input-output curve. 2) When the common emitter circuit is examined under DC conditions, what is the positive effect of the RE resistor on the circuit? 3) What is the effect of junction capacity and diffusion capacity, which arise due to the semiconductor structure of the transistor, between the junctions at high frequencies? 4) How must be gain calculated in cascade-connected multi-layer transistor amplifiers? 5) In Figure 8, calculate the second layer output voltage VO2 if the first layer voltage gain AV1 = - 40, the second layer voltage gain AV2 = -50, and the first layer input voltage VI1 = 1mV. 6) Formulate the total voltage gain of the amplifier circuit in Figure 7 depending on the input impedance and current. Figure 8 7) In practice, the change of earning by frequency is usually given in decibels. Learn about decibel concept and related calculations. Recapture the decibel gain (db) -frequency graph of the normalized gain-frequency curve shown in Figure 9. (Normalized gain is obtained by dividing the gain value Av in each frequency band by the gain value in the middle frequencies AvOF.) Figure 9 7

8 RESULTS OF THE EXPERIMENT 1 1.a) Plot the gain graphs obtained with and without CE. Do not forget to write units to the axes. With CE Without CE 1.b) Comment on the gain curves for both cases. 2.a) Plot the gain-bandwidth curve obtained by changing the frequency of the signal generator. 2.b) What is the bandwidth of amplifier? Result and Comment: 8

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