EE462G: Laboratory Assignment 7II. BackgroundEE462G: Laboratory Assignment 7BJT Common Emitter AmplifierbyDr. A.V. RadunDr. K.D. Donohue (6/2/04)Department of Electrical and Computer EngineeringUniversity of KentuckyLexington, KY 40506 (Lab 6 report due at beginning of the period) (Pre-lab7 and Lab-7 Datasheet due at the end of the period)I. Instructional Objectives- Understand the basic operation of the bipolar junction transistor (BJT)- Apply a DC load line to establish a DC operating point- Perform a small signal analysis of a BJT circuit to compute small signal input and output resistance and gain- Experimentally measure small signal input and output resistance and gainII. BackgroundA transistor (MOSFET and BJT) can be used to amplify a time-varying input signal (AC), after DC voltages are added to the AC input to ensure that the transistor is operating in its linear region (saturation region for a MOSFET, forward active region for a BJT). Transistors are nonlinear devices that can be approximated with linear models overcertain regions. DC levels in the transistor circuit can be set to bias the AC signals at the transistor terminals operation in the linear region of the transistors voltage-current characteristics. The transistor circuit DC currents and voltages are referred to as either the DC operating point, quiescent operating point, or bias point. Once a transistor is biased in its linear region, its currents and voltages will vary linearly with changes in the input signal as long as they stay within the transistor's linear range. It is assumed that the variation of the transistor's input signal and other currents and voltages are small enough so as not to perturb the system into nonlinear regions of operation (triode or cutoff for a MOSFET, saturation or cutoff for a BJT).Bipolar Junction Transistor (BJT) BiasingFigure 1 shows a simple common-emitter bipolar junction transistor (BJT) amplifier biasing scheme. The time varying part of the input signal is omitted at this time to focus on the bias point. In the actual circuit operation the input signal consists of an AC signal added to the VBB. The transfer characteristic (output as a function of the input) for this circuit can be derived as: fBBBCfCCout VVRRVV ,(1)where f is the current gain between the collector and base current (Ic / Ib), and Vf is the internal voltage drop over the base-emitter junction (VBE). At the operating point, VBB and Vout are the DC or quiescent values of the input and output voltages. Ideally, for a given VBB, Vout should not vary if the temperature varies or if different transistors of the same type are used. Unfortunately the BJT's current gain f cannot be controlled well during manufacturing and it also varies with temperature. For the 2N2222 BJT transistor, manufacturers specify that f may be anywhere from50 to 150. Thus, a circuit biased correctly for one 2N2222 transistor may not be biased correctly for another 2N2222 transistor. A more robust biasing scheme can be developed using feedback through an emitter resistor so that the BJT's quiescent operating point is more resilient to changes in f.Figure 2 shows a more robust design with resistor RE placed in the emitter branch of the circuit. The DC analysis of this new circuit for the collector current results in:  IV VR RCf BB fB f E 1. (2)Note that if (f + 1)RE >> RB and f >> 1, the collector current can be approximated as  IV VRCBB fE, (3)which is independent of f. VCC VBB C B E Vout + - RC RB IB IC VCC VBB C B E Vout + - RC RB RE IB IC Fig. 1. Basic common emitter amplifierbiasing.Fig. 2. Basic common emitter amplifier with reduced f sensitivity.The schematics for these circuits indicate that two power supplies are required, one for VBB and another for VCC. Thecircuit in Fig. 3 shows a scheme where only one power supply is required. The Thévenin equivalent for the circuit consisting of VCC, R1, and R2 in Fig. 3 results in the biasing circuit of Fig. 2, where VBB = Vth and RB = Rth. With theseThévenin equivalents substituted into the circuit, the circuit is identical to the circuit in Fig. 2 with the exception thatthe input bias voltage VBB is now dependent on VCC. The VBB voltage is now controlled by the proper choice of R1 andR2. This eliminates the need for a separate power supply to control VBB.VCCCBEVout+-RCR2RER1Fig. 3. Basic common emitter amplifier biasing with reduced f sensitivity and employing a single DC voltage.Choosing the resistors R1 and R2 such that RB << (f +1)RE is equivalent to making the current through R1 and R2 large enough that the BJT's base current can be neglected in comparison. The base voltage is thus determined only by VCC and the R1 and R2 voltage divider. The DC Operating point of this circuit is stable for two primary reasons:- The base voltage is determined primarily by the voltage divider R1 and R2 and is effectively independent of the transistor parameters (especially f).- The emitter resistor RE stabilizes the DC operating point through negative feedback. If f increases for any reason, such as temperature change, the subsequent rise in emitter current will increase the voltage drop across RE, thereby increasing VE and VB (since the drop across VBE is a constant). The voltage drop across RB is then smaller, causing a drop in IB that counteracts the attempted increase in IE. Note that the BJT’s base to emitter voltage (its input) is equal to the input voltage minus the voltage across RE, which results in negative feedback.Once the circuit in Fig. 3 is biased, it may be used as a voltage amplifier by connecting an input signal source to the base of the transistor, and connecting a load to the collector. These connections are coupled through a capacitor, as shown in Fig. 4, in order to prevent the source and load from altering the BJT’s DC operating point. The capacitor Cin between the signal source and base voltage of the transistor keeps the DC voltage at the transistor base from being affected by the AC source’s low impedance. In the same way, capacitor Cout ensures that the added load resistance does not change the DC voltage at the collector. These capacitors perform this function by being open circuits at DC. At the frequency of the small AC signal voltages and currents the capacitor values are chosen so they have a low impedance and allow the AC signals to pass