ECE 3110 - Full Lab Report 2

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Dec 6, 2023

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BJT Common-Emitter Circuit Voltage Gain October 31, 2023 ECE 3110 – 003 Electrical Engineering Laboratory III ABSTRACT: This experiment is intended to show how altering the bias of the circuit and the resistor values inside it can alter the voltage gain of a C E bipolar transistor circuit. The function of the emitter bypass capacitor C E will also be demonstrated. Furthermore, it will be illustrated how the Q-point is determined by the base resistors and how the Q-point impacts the relationship between the input and output wave shapes.

INTRODUCTION: One common electronic circuit architecture for amplifying tiny signals is the common- emitter ( C E ) transistor amplifier. It is commonly applied for small-signal AC and DC amplification, and it makes use of a bipolar junction transistor (BJT). When there is no input signal for the transistor, the common-emitter circuit’s DC function includes setting a stable operating point or Q-point. The DC values of collector current ( I C ) and collector-emitter voltage ( V CE ) are determined by this Q-point. The transistor is kept in the active zone and prepared to amplify signals thanks to DC biasing. The C E amplifier also amplifies weak AC impulses and sends them to its base terminal. The transistor operates differently when the input signal is placed on the DC biasing point (Q-point). The ratio of the AC output voltage to the AC input voltage, or V O V I , determines the voltage gain of the C E amplifier. By modifying the DC biasing resistors, the Q-point can be changed, which in turn affects the transistor’s operating voltage and current ( I C and V CE ). On the transistor’s characteristic curves, if the Q-point is moved excessively to the left, the transistor may enter cutoff or saturation states, which would restrict linearity and signal amplification. To guarantee that the transistor runs in its active region and enables efficient signal amplification with ideal gain and linearity while preventing saturation or cutoff, the Q-point must be adjusted correctly. Changing the values of the emitter and collector resistors has an impact on the circuit’s voltage gain. A BJT’s collector current ( I C ) is determined by the transistor’s  , temperature, and circuit components. However, selecting biasing resistors so that the quiescent (DC) collector current stays consistent, independent of transistor temperature, is necessary for successful amplifier design. The collector current and voltage gain are kept reasonably constant by choosing

the appropriate biasing network and resistors. The emitter resistor, RE, and the values of the bias resistors, R1 and R2, are the essential components that stabilize the circuit against variations in the transistor. The stability of the circuit in response to variations in the transistor’s  is influenced by essential components, namely the emitter resistor ( R E ) and the bias resistors ( R 1 and R 2 ). To preserve the high gain of the common emitter ( C E ) circuit, a common practice is to bypass the emitter resistor with a sizable capacitor. The introduction of an emitter resistor diminishes the AC voltage gain at the AC component ( i C ) is obliged to pass through RE to reach ground. Consequently, RE hinders i C , and the negative feedback voltage across RE diminishes the AC gain. To counteract this undesirable AC response while retaining effective control over I C (collector current) and the stabilizing impact of RE, a capacitor is connected across RE to short-circuit the AC current ( i C ) around RE to ground. This capacitor, usually denoted as C E , is typically of substantial magnitude, such as 10  F or greater, facilitating the majority of the AC current to flow through RE to ground. I C still traverses through R E as the capacitor functions as an open circuit for DC, thereby maintaining the stabilizing effect of R E . The investigation in this laboratory will explore the impact of the absence of C E , revealing that the voltage gain is determined by the ratio of R L (total AC load resistance) to R E . OVERVIEW / BACKGROUND: Before conducting the lab, review the dc analysis methods for determining a common- emitter circuit’s Q-point and how changing the ratio between two base resistors ( R 1 and R 2 ) affects the load line’s motion and base voltage. As prelab, LTSpice simulations were

performed of the circuit in Figure 1, using the resistor values listed in Table 1, both with and without C E . Assume sinusoidal input VS = 0.02Vp-p at 1 kHz, the transistor is a 2N3904, and all capacitors are 22  F or greater. The simulated results were recorded in Table 1. Important formulas: A V = V O − pp V S − pp , which was used to determine the voltage gain.  = I C I B V CE = V CO − V EO I C ≈ I E = V EO R E V CE (max) = V CC I C ( max )= V CC R E + R C Table 1: List of Resistors for Prelab and Simulated Data With C E Without C E R C (  ) R E (  ) V OMin (mV) V OMax (mV) V OMin (mV) V OMax (mV) 3.9 k 1 k -915.3412 756.27914 -19.886941 20.218734 3.9 k 200 -265.35281 218.92653 -86.002651 89.18741 3.9 k 10 k -32.985416 25.446516 -1.894901 2.183624 1 k 1 k -422.890132 345.56552 -37.23240 39.28542 20 k 1 k -15.448301 446.70943 -7.287621 33.058711

Figure 1: Common-Emitter circuit with emitter bypass capacitor EXPERIMENTAL PROCEDURE and RESULTS: Equipment used: - 1 NPN Transistor, 2N3904 - Curve Tracer function on Waveforms - 1 Decade Resistance Box - NI-ELVIS workstation - Resistors (2.2k, 220, 16k) - Capacitors (22  F) - Lab Kits - AD2 Part 1 Procedures: During this part, the proper Q-point was established. Using the curve tracer, a Q-point closest to V CE = 2.5 and I C = 1 mA was determined which was V CE = 2.5096 and I C = 1.4256 mA. Additionally, I b = 0.01064 mA and V bE = 0.69988 and  = 1.4256 0.01064 = 133.984. Figure 2 shows the screenshot of the determined Q-point on the transistor characteristics graph.

Figure 2: Screenshot of Q-point found on Waveforms Using Figure 3 as reference, the circuit was constructed with V CC = 5 V . The DMM was used to measure V CO and V EO and got a measurement of V CO = 3.115V and V EO = 0.184 V. V CE was determined as V CO − V EO = 3.115 – 0.184 = 2.931 V and I C ≈ I E = V EO R E = 0.184 220 = 0.83636 mA. Figure 4 portrays the transistor characteristic with the determined Q- point using the DMM and drawn load line. . Figure 3: Common-Emitter Circuit for Q-point measurement

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