The Bipolar Transistor and the Long-Tailed Pair Experiments - Term Paper Example

Summary This report describes the experiments undertaken to investigate the characteristic of the bipolar junction transistor and the Long Tailed Pair. The minority carriers in the base-emitter region promote the flow of current when the collector-base junction is reverse biased. The function hFE connects the base current to the collector current. It is the ratio of the collector current to the base current. In the first experiments, the basic characteristics of the transistor are determined. The transistor model is shown below. The biasing of a transistor amplifier circuit sets the DC voltage and current to the right levels so that a signal can be accepted and amplified by the circuit. With the bias current set properly, the transistor amplifier is ready to receive an input from the signal generator and amplify it properly. A small signal current through the bas will result in a large amplifier signal coming at the output of the transistor. This amplification is referred to as Gain and is the ratio of the output voltage to the input voltage. The proper bias point that is somewhere between the higher and lower limits of the input current and is the quiescent operating point, or Q-point. In closed loop amplification, the ratio of the amplification is equal to the ratio of Rc of the Collector to the Re of the emitter. A long-tailed pair is a pair of transistors constructed identically so that they operate together at the same temperature. The SS2210 dual transistor is such a good example. The ac condition is used to show that the output voltage of the long tailed-pair is a difference voltage. The current mirror is a circuit used as a source and ensures the tail current is constant even if the input voltage has a common-mode offset. The Bipolar transistor has a higher switching speed and a larger output drive current compared to the CMOS transistor. The higher speed is attained at the expense of higher power dissipation. They are therefore not suitable for application where higher operating temperatures are common. This has been a limitation in their use in the high-power-high-frequency signal amplification systems used in space satellite communication. Procedure In these series of experiments we use several circuits to determine the properties of a bipolar junction and the long tailed pair. To investigate the hFE factor of a transistor we switch the power supply and press key 25. Next we adjust power supply to 12v and limit the currents to 100mA. We build the following circuit in the plug-in breadboard using the BC547C Transistor. We connected the +12 supply as above with Rb equal to 100 kΩ. We use the Agilant multimeter to measure the voltage across Rb. We then calculated Ib using the formulae (Vrb/Rb). W e read the current displayed on the power supply. We then calculated the factor hFE using hFE=Ic/Ib=(I-Ib) We repeated this procedure of getting hFE for random Ic from 54 mA to 1mA In the next experiment, we investigated the bias circuit for bipolar transistor. We chose the value of Ic=1mA and then set Vce value to half that of the supply, To get 6v. We divided the remaining 6v by having Vre=1mA giving us 5kilo ohm and Rc giving us (Vrc/1ma)=5 kΩ. This could be alternatively be obtained by two 10kilo ohm in parallel. Vb=VRE+Vbe Ib=Ic/hFE=1mA/hFE The chosen bias current was equal to 20*Ib. The R2 was calculated by Vb/Ibias. To get R1 We calculated it using the formulae R1=(12v/I bias)- R2 (approximate). Alternatively a more precise value R1=(12V-Vb)/(1bias +Ib) We corrected the count to the power supply and measured Vre, Vrc and Vb replacing r1 and r2 with suitable values from E12 range. Small signal amplification of bipolar transistor We modified the circuit used to investigate the bias current of the bipolar transistor to include the bias current of the bipolar transistor to include the potential dividers, resistors and two electrolytic capacitors. We adjust the signal generator to give a sine wave of 1kHz and 100mV peak to peak. We then connected the signal generator to the plug-in bread board using a bnc the 4mm lead. We connected one channel of the oscilloscope to Vin and the other to Vout. We adjust the voltage output of the signal generator so that Vout from the transistor came to be about 2v peak to peak value of the voltage at Vin and Vout. We then calculated the actual voltage amplification using the formulae. Av=Vout(p-p)/Vin(p-p) Then we calculated the theoretical voltage where Amplification=gm*Rc =39*(lcmA)*(Rc kΩ) We finally recorded the actual and theoretical values of voltage amplification. The circuit used is shown below. Close loop amplification of bipolar transistor We then modified the previous circuit by removing the potential divider and the emitter decoupling capacitor. This was followed by us adjusting the signal generator to about 200mV peak-to-peak voltage. Then we connected the signal generator to the circuit Vin. Using the oscilloscope, we measured the values Vin and Vout. Then we calculated the closed loop voltage amplification. The circuit was then analysed for the following values hFC at collector currents of 10mA, 1mA Long-Tailed pair The next experiment we performed was a Test circuit to set the quiescent condition. We constructed the circuit on a bread board making sure nothing was connected to any output terminal of the power supply. We turned the power supply on. To show voltage and current in the lower on the display we pressed the output on/key. Then we pressed track for a few seconds so that the mode was confirmed n the display. Then we pressed +25Vand adjusted the wheel until 15V appeared on the display. Then we pressed display limit followed by voltage current and adjusted the current limit to 0.1a. Then we pressed 25V to indicate that -15V tracking mode is selected. The power supply was now set up correctly to ± 15 volts at a maximum of 0.1A. Then we pressed output on/off so that the display read OUTPUT OFF. Then we connected the power supply to the bread board using the special red, black and blue twisted wire, observing the colour convention where red=+5v, black=0v and blue=-5v. We pressed output on/off so that the display read voltage and current. Then we connected the black lead of the Agilent DMM to zero volts and measured the quiescent condition as follows: Vx=-0.53V Vc1=10.1V Vc2=10.1V Actual voltage across Rx=Vrx=14.46 Actual value of Ix=Vrx/Rx=14.46/15= 0.964mA The measurements were within the allowed range of -0.7 for VX, Vc1 and Vc2 at 10v. The circuit was therefore ready to be tested under a.c condition. With both collectors at the same voltage, the differential output is zero in the quiescent condition. Test Circuit: a.c condition We switched on the Agilant function and made sure it was in the high Z mode. We adjusted the output for sine wave to become 1 kHz, 100mv peak to peak operation. We connected the function generator to Vin using a bnc o 4mm plug lead. We used one channel of the oscilloscope to check that Vin is correct in amplitude. We selected DC input coupling on both oscilloscope channels and adjusted the virtual sensitivities to 2V per division. Next we positioned the two traces superimposed near the bottom of the screen. Then we connected the two oscilloscope probes to the outputs Vc1 and Vc2. The two sine waves displayed were in the top of the screen, in anti-phase and exactly superimposed. We then selected the settings key and adjusted the scale to be true is positioned somewhere in the lower part of the screen. We measured the peak to peak voltage of the new trace. The measured value of amplification was equal to 11*Vout(p-p)/Vin (p-p) Theoretical value of amplification=gmRc where Rc=10 kΩ and gm is given by gm=Ix/2Vi =actual value from ()/ 52mV The current mirror A current mirror is useful in setting the tail current for LTP as it keeps the tail current constant. The additional advantage is that it can be used to set other currents in the operational amplifier. When two transistors are shown connected as shown, their voltage become identical. One of the transistors is connected base to collector so that the base currents produce an error on one connection. A large gain of say more than 100 makes the error in the two currents to be less than 2% an amplitude level. The two transistors must be fabricated side by side on the same silicon chip. This ensures they are identical and have same temperature. Test circuit for Current Mirror We wired the long-tailed pair to give the above circuit. With the emitters connected to zero volts. We use the 15 kilo ohm tail resistor for R2 which sets the current to approximately 1mA We try resistors of 1 kilo Ohm, 5 and 10 kΩ in the r1 position and measure the voltage across R1 in each case. We then calculate Ic1 Ic1=voltage across R1/ Value of R1 The Ic1 should be the same value for all three resistors. TR1 is a true current source controlled by the current Ic2. In the last part of the experiment, the long tailed pair was reconstructed using the current tailed mirror as the source of the tail current Ix. This ensured that the tail current was constant even though the output voltage was a common-mode offset. Demonstration of common Mode Rejection We rewired the precious circuit for the Current Mirror and used the transformer to isolate the function generator. We set the function generator to sine wave 10 kHz 100mV p-p operation. We set the 6V output of the power supply to give 0V, then we turn the power supply to ‘output on’ and observe the wave forms at Vc1, Vc2 and the difference is output as described previously in the test circuit for current mirror. We adjust the function generator amplitude to around 2V p-p at the output of the long tailed pair (differential Vout) We then varied the voltage output of the 6V power supply ONLY and observe the effect on the voltages at the collectors and differential Vout. We reversed the terminals on the 6V power supply and repeat the previous step and then removed the underground circuit from the current mirror. We connect a 15k resistor from Vx to -15V. We adjust the 6V output in both polarities as previously described. The fist adjustment has no effect on the different Vout, in the second however Vout increased as 6V output was increased in positive polarity and decreases as the 6v output is increased in the negative polarity. The tail current was not constant and the value of the gm and amplification therefore vary. This proved that the common mode rejection requires a constant tail current. Results Bipolar transistor amplifier Measurement of hFE Ib=VRb/Rb Rb=100kilo ohm hFE=I/Ib I=Read value from display=Ic Other values are in the table below. 1.3 Measurment of hFE Vin - Vbe Ic mA Ib hEF 12-0.7=11.3 1 = 3.29µA =303.95 12-0.7=11.3 2 =4.04µA =495.050 12-0.7=11.3 5 = 10.11µA =494.559 12-0.7=11.3 10 = 20.33 µA =500 12-0.7=11.3 20 = 43.34 µA =461.467 12-0.7=11.3 30 = 68.64µA =437.062 12-0.7=11.3 40 = 112.11 µA =356.792 12-0.7=11.3 50 = 226.93µA =220.332 12-0.7=11.3 54 =300.26 µA =179.844 Bias circuit for bipolar transistor Vb=11.3 Ib=2.100 Ibias= 202.100= 4.20 R2=39.52K ohms R1==234.467 K ohms Ic=1mA Vce= Vrc - Vrc = 5.938 -1.156 =4.8V Vre=1.156V Vrc=5.938V Vb=1.75V Small signal amplification of bipolar transistor Actual voltage amplification =Vout p-p/Vin p-p A=148 Theoritical Voltage amplification =gm*RC ATh= gm×Rc = 39 × (Ic in mA) × (Rc in KOhm) ATh=39×1×5=195 The wave forms below represent an input signal in the lower part of the display with its amplified form in the upper part of the display. The amplified signal is seen to be a compound wave form due to noise picked from the circuitry and the surroundings of the circuit. The theoretical value is more than the actual value. Losses resulting from heating of devices used and resistance of connecting wires may be the cause of the difference in the theoretical and the actual amplification. Closed loop amplification of bipolar transistor The waveforms below represent the amplification of the bipolar transistor where the amplified output is in anti-phase to the original input wave. The noise factor is shown to be cut out in this amplification. Closed loop amplification= Vout p-p/Vin p-p The closed-loop voltage amplification = 4.20 Operational amplifier Long-Tailed pair Transconductance gm = differential output current/differential input voltage gm=IC1-IC2/Vin1-Vin2 gm=tail current Ix/ 2VT gm=19m ohm-1 Test Circuit: quiescent condition The waveforms below display the characteristic quiescent condition, where the wave forms all start at the same point that is they are in phase even though their amplitudes differ. The quiescent condition set the circuit to have a reference starting point for its amplification. VC1=VC2=10.1V VX=-0.53V IX=0.964Ma A.C. test condition The two traces shown in the display waveforms below are equal in amplitude but are in anti-phase. The waveform in the upper portion of the display shows combined amplitude of the two waves. This output is a differential voltage. Measured value of amplification =11*Vout(p-p)/Vin(p-p) =11*3V/3.5V Theoretical value of amplification Ath =gmRc Rc=10kOhm gm=Ix/2VT gm=19 m ohm-1 Ath=10*19=190 The Current Mirror 2.6 Test Circuit for Current Mirror hFE=500 Ic2=Ic1 Ib=Ic2/500 (15-0.7)V/15kohm =1mA The table below shows that collector currents are equal as expected different values of R1. R1 Voltage across R1 Ic 1k 1.077V =1.077mA 5k 5.207V =1.041mA 10k 10.012V =1.001mA Long- Tailed Pair with Current Mirror The wave forms below represent the output of the Long-tailed pair. The wave forms are in ant-phase but of equal amplitude. The wave form in the lower part of the display is the differential output of the two waves. Its amplitude is twice that of the long tail pair individual amplitudes. Demonstration of Common Mode Rejection The wave forms of transistor at VC1 and VC2 are shown below. They are equal in amplitude and in anti-phase. The power output voltage is 0 Volts. 0v The wave forms below show the two waves at VC1 and VC2 superimposed on each other but in anti-phase. The second wave form on the upper half of the display is the differential output of the circuit. The circuit is at zero volts output. 0v The wave form below is at 6 volts and positive polarity but there is no change in the amplitude of the waveform. Varying the 6V output had no effect on the differential polarity. Reverse polarity The wave form below is shown to have the same differential polarity even though the circuit is in reversed polarity. Varying of the 6V power supply output had no effect on the differential output. 0v Varying the 6V power supply output to -6V showed no difference in the differential output. The circuit was in reversed polarity. This output is shown in the wave display below -6v Common mode rejection without circuit mirror With the current mirror replaced with the 15k ohm resistor, there was a change in the differential voltage when the‚ 6Vpower supply output was varied. Decreasing the output resulted in decrease of the differential voltage. 0v When the 6V output was increased, the differential input increased. The increase was in proportion to the increase in the 6V output. The change is shown below. 6v Decreasing the 6V output gave a decreased differential voltage. Varying the 6V output by increasing it or decreasing it resulted in a corresponding increase or decline. This is shown below. -6v Discussion Transistors are three terminal devices that can function as electronic switches or can be used as signal amplifiers. The transistor unlike the diode which has two layers, it has three layers with the base in the middle. The arrow is on the emitter terminal and points to the N- doped material. When the base/collector junctions are reverse biased and the emitter/base forward biased, the majority current carriers (electrons), cross the forward biased junction and combine with majority carriers (holes) in the base to cause base current. Compared to the emitter, the base is physically thin and lightly doped; the result is that there are a large number of electrons at the base that do not combine with the hole which are limited. The electrons end up falling under the collector voltage supply Vcc. And since the base collector junction is reverse biased and majority current carriers are prohibited from crossing the junction. A junction that is reverse biased for majority carries is forward biased for minority carries which in this case are the electrons. The electrons in the base therefore being the minority find it easier to cross the base collector junction. Once in the collector they attracted by the positive terminal voltage supply. This is then recorded as current and it is huge compared the one flowing from the emitter to base. This forms the basic functionary of the transistor as a switching and amplifying device. The amplification is as a result of the large current that is proportional to the small base current while the switching property is as a result of the current flowing through the collector triggered by the small current through the base. A long-tailed pair A long-tailed pair is built with two bipolar transistors. It is most important for its low noise and, with appropriate trimming has a low voltage offset. When such a stage is trimmed for minimum offset voltage it is inherently characterised by minimum offset drift. Its chief weakness stems from the proportionality of the emitter and base currents of the transistors, whereby if the emitter current is big enough for the stage to have a realistic bandwidth, the base current-and hence the bias current-will is relatively large especially for high-speed ones. Conclusion The bipolar junction transistor has revolutionised how electronic circuits function. The only limit they have had is in amplification of the high powered signals as used in telecommunication especially space satellite communication. High-power, high-frequency operation, which is used in microwave links and over-the-air broadcasting, is better achieved in vacuum tubes. They have improved electron mobility in a vacuum. Where their performance is greatly reduced and noise is increased. However in majority of applications, the transistor has revolutionised how circuits are designed. The combination of the long-tailed pair and current mirror has seen low frequency signals be amplified with very minimal noise levels. This has made the bipolar transistors devices of choice in low-power-high-fidelity circuits. References Cooke, M. J. 1990. Semiconductor devices / Hemel Hempstead : Prentice Hall Greig, James.1970. Electrical engineer. London: H.M.S.O. Gayakwad, Ramakant A. 1993. Op-amps and linear integrated circuits.(3rd ed). Englewood Cliffs, N.J. : Prentice-Hall. Johnson, David E. , Jayakumar, V. 1982.Operational amplifier circuits: Published: England Cliffs N.J. : Prentice-Hall Orton, J. W., Blood, P. Electrical. 1992. Characterisation of semiconductors: majority carrier properties and electron states / London : Academic Press. Ohring, Milton, 1998. Reliability and failure of electronic materials and devices. San Diego: Academic Press. Till, William C. , Luxon, James T. 1982 Integrated circuits : materials, devices, and fabrication. Englewood Cliffs, N.J. : Prentice-Hall Zissos, D. , Bathory, J. C. 1984.System design with microprocessors. (2nd ed.0) London : Academic Press. Read More