The Design, Implementation, and Test of Bipolar Transistor Amplifier - Term Paper Example

Name Course Institution Instructor DATE Contents Bipolar Transistor Amplifier 19 1 Introduction 4 EG-251 Practical Circuits: Long-tailed Pair 5 The SSM2210 dual transistor 5 The Long-tailed pair 5 The current mirror 6 Procedure 7 Bias circuit for bipolar transistor 7 Small-signal amplification of bipolar transistor 8 Test circuit: quiescent condition 10 Test circuit: a.c. condition 11 Test Circuit Current Mirror 11 Demonstration of Common Mode Rejection 12 Results and calculations 13 Bias circuit for bipolar transistor 14 Small signal amplification of bipolar transistor 14 Closed-loop amplification of bipolar transistor 15 Test circuit: quiescent condition 16 Ac condition 16 Discussion 18 Test circuit: a.c. condition 18 Common mode rejection 18 Conclusions: 18 References 20 Introduction This report describes the design, implementation and test of a common-emitter amplifier using BC547C NPN BJT transistors. By use of bipolar transistors small-signal analysis of differential amplifiers was performed. The report will also show calculations done on both the differential gain and the common-mode gain. Other characteristics are investigated on bipolar transistors during exercise like common-mode rejection. Basically, a transistor is made of two diodes connected back to back as show below. Design current sources such as current mirrors sources for biasing of amplifiers Minority of carriers created at the base-emitter region of the transistor are responsible for the amplification of current in the transistor. Minority carriers promote the current flow in the reverse biased collector-base junction. A model of a tarnsistor consists of p-n junction which represent base-emitter and a dependent current source which represent the path from the collector to the emitter. This is shown below. Current gain is the ratio between collector current to the base current. The first part of this experiment will determine the current gain hFE and base/emitter diode voltage characteristics. EG-251 Practical Circuits: Long-tailed Pair The SSM2210 dual transistor Along-tailed pair can only be constructed using well-matched transistors at the same temperature. The integrated circuit is made by Analog Devices incorporated and intended for use in low-noise amplifiers such as microphone preamplifiers and other critical applications. The Long-tailed pair The most distinct feature of operational amplifier is the long tail pair, which provides the differential input stage. It is possible to construct an LTP using bipolar transistor or field effect transistors, but the exercise will only cover bipolar transistor circuits. The amplification of a long-tailed pair is normally quoted as transconductance, gm, which is the ratio of differential output current and differential input current. The transconductance has the dimensions of reciprocal of resistance or conductivity. An analysis of the transistor equation shows that gm depends only on the tail current and certain fundamental constants: gm = (tail current I×)/ (2VT) where VT = kT/q = 26 mV at room temperature The voltage amplification of the test circuit is given by the product gmRc. The current mirror The tail current for the LTP can be set by connecting a resistor between the common emitter and the negative voltage trail. The current will however change slightly as the voltage input move up and down with the corresponding transconductance changes. The use of current mirror technique is much better, since it keeps the tail current constant and can be used to set other currents in operational amplifiers. The base-emitter voltages of the two transistors shown above are identical, and therefore their collector current will be identical. One of the transistors is connected collector to base and therefore the base current produce an error on one connection. If the gain of the transistor is greater than 100, then the error in the two currents will be acceptable since it will be less than 2%. For the current mirror to work, the two transistors should be identical and at the same temperature. This can only be achieved if the transistors are fabricated side by side on the same chip. The long tailed pair was reconstructed at the last part of this experiment. This was done using the current mirror as the source for the tail current Ix. This makes the tail current to be constant even if the input voltages have a common-mode offset. Procedure The experiment was started with measurement of current gain. The experiment was set on a breadboard as shown below. The circuit was then connected to a +12V supply with initial Rb = 100 kohms. Agilent multimeter was used to measure the voltage across Rb. Collector current, base current and base-emitter voltage was recorded on the table. Care was taken not to let transistor overheat as it could be destroyed. Bias circuit for bipolar transistor To get the value of Re and Rc, the value Ic =1mA and Vce = Half supply voltage = 12V/2 = 6V. the remaining value is split as follows; Vre = 1V, Vrc = 5V. Then Re = Vre/1mA = 1 kohms and Rc = Vrc/1mA = 5 kohms. 2 Kohms and 10 kohms are chosen for Re and Rc respectively. The circuit obtained is as shown below: The suitable values of R1 and R2 are chosen as follows. Vb = Vre +Vbe = 1V + (the value of Vbe obtained above). Ib = Ic/hFE = 1mA/ 9the value of hFE). To choose I bias = 20*Ib, then R2 = (Vb/I bias). R1 = (12V-Vb)/ (I bias + Ib). The circuit was constructed on a plug-in breadboard as follows. Values of R1 and R2 were selected from the E12 range, which runs: 1, 1.2, 1.5, 1.8, 2.2, 2.7, and 3.3, 3.9 and so on. 12V power supply was then connected and values of Vre, Vrc, Vce and Vb were measured. Small-signal amplification of bipolar transistor The 1k and 100 Ohms resistors are connected as a potential divider as shown below. The actual value of input voltage is Vin = Vsg/11 The transistor circuit was modified to include the potential divider resistors and two electrolytic capacitors. This was done as follows: the signal generator was adjusted to give a sine wave of 1 kHz and 100 mV peak-to-peak. The signal generator is then connected to the plug-in breadboard using a bnc to 4 mm lead. The channel of the oscilloscope is connected to Vin and the other channel to the Vout. The voltage output of the signal generator is adjusted so that Vout of the transistor was about 2 V peak-to-peak. The peak-to-peak value of the output voltage is measured at Vin and Vout. Using the values obtained voltage amplification was calculated. The circuit was further modified by removing the potential divider and the emitter decoupling capacitor as shown below. The output voltage of the signal generator is then adjusted to about 200 mV peak-to-peak. The signal generator is connected to the circuit at Vin. By us of the oscilloscope Vin and Vout peak-to-peak values are measured. Closed-loop amplification was then calculated using the formula, C.L.A = Vout/Vin. Test circuit: quiescent condition The circuit is connected as shown below on the plug-in breadboard. Care is taken to make sure that nothing is connected to any output terminal of the power supply. It is then powered on using the push-button in the lower left corner. The oscilloscope was then set so that the observations can be made easily. The power is set up correctly to deliver +/-15 V at a maximum of 0.1A. The display was put off and the power supply is connected to the breadboard using the special red, black and blue twisted wire, also observing the colour convention red = +15, black = 0V, blue = -15V. The black lead of the Agilent DMM was connected to zero volts and the quiescent conditions were then measured. Test circuit: a.c. condition The Agilent function generator is switched on making sure that it is in high Z mode. The output for the sine wave is then adjusted to 1 kHz, 1000 mV p-p operation. The function generator is connected to Vin using a bnc to 4 mm plug lead. By using one channel of the oscilloscope Vin is checked if it is correct in amplitude. DC input coupling is selected on both oscilloscope channels, and then the vertical sensitivities are adjusted to 2V per division. The two oscilloscope probes are now connected to the outputs Vc1 and Vc2. Test Circuit Current Mirror The long –tailed pair circuit was rewired as shown below. 15k “tail” a resistor was used for resistor R2 then for R2 1k, 5k, and 10k was used, while taking the measurements of voltages across R1in each case. Demonstration of Common Mode Rejection The circuit was rewired using the transformer as shown below. The generator was then set to a sine wave of 10kHz, 100mV p-p operation.6V output power supply was set to give 0V and then turned to “output on” and observations were made on the waveform at Vc1, Vc2, and the difference output. The function generator amplitude was adjusted to about 2 V p-p of the output of the long-tailed pair. This is also called the differential output. The voltage output of the “6V” power supply was then varied and the effect on the voltage observed at the collectors and differential Vout. The terminals of the “6V” power supply were reversed and the same process repeated. The integrated circuit forming the current mirror was removed and 15k resistor was connected from Vx to -15 V. “6V” output are adjusted in both polarities. . Results and calculations Psu 1- 100mA= I; V= 0V Psu 2- 100 mA=I ; V= 5V The table obtained is showed below Vin [V] Vbe [V] Ib[µA] Ic [mA] hFE Vin-Vbe [V] 0.88 0.644 2.36 1 423 0.236 1.1 0.659 4 2 500 0.441 2.9 0.697 22 10 454 2.105 24.39 0.749 236 50 211 23.641 Bias circuit for bipolar transistor Vb= Vre + Vbe = 1v + 0.644 V Ib= Ic/hFE = 1mA/423= 2.36uA I bias= 20 x Ib = 47 uA The table below was obtained for R1=35 KΩ, R2= 210 KΩ, 12V: Vre [V] Vce [V] Vrc [V] Vb [V] 0.972 6.204 4.818 1.6 Small signal amplification of bipolar transistor Vsg = 160 mV PP > Vout = 2.10V PP Vin (actual) = Vsg/11 = 0.16/11= 14mV PP For oscilloscope Vsg = 231 mV Actual Voltage amplification = 2V/14 mV = 142 Theoretical amplification: gm x Rc = 39 x IC x RC = 39 x 1 mA x 5 T ohms = 195 IC -> amp meter = 1 mA 2vPP output on channel 2, input from function generator Vsg=160mV, below: Closed-loop amplification of bipolar transistor Closed loop amplification Av = Rc/Re =5k/1k = 5. The amplification is equal to the ratio of Rc and unbypassed Re. Signal of voltage ammeter Vin = 600 m VPP f = 1 tMc Closed-Loop amplification = Vout (p-p)/ Vin (p-p) = 2.43/663 = 4.43 Closed loop amplification of bipolar transistor was displayed as follows: Test circuit: quiescent condition Theoretical amplification: gm x Rc = 39 x IC x RC = 39 x 1 mA x 5 T ohms = 195 IC -> amp meter = 1 mA Test circuit quiescent condition: Vx= - 0.558V Vci= 10.198V; Vc2= 10.205 V Rx -> Vrx = -14.444V; Ix= Vrx/Rx = 14.444V/15t = 0.36 mA Ac condition Measured value of amplification = 11 x Vout(p-p) / Vin(p-p) = 11 x 1.5V/ 0.1 = 165 Theoretical value of amplification = gm Rc, (but gm =1x/2VT) = Ix/ 2VT x 10 k = (0.96 mA/ 2x 26mV) x 10K = 184.6 Test circuit: a.c. condition was displayed as follows: Discussion Test circuit: a.c. condition The circuit worked properly since the two sine waves were seen on the top half of the screen, in antiphase and superimposed as shown below. This shows that the output voltage of the tailed pair is a difference in voltage. Common mode rejection By adjusting “6V” there was no effect on differential output, this indicate that the circuit is correct. When the “6V”output of both polarities was adjusted, the amplitude of the differential Vout increased as “6V” output increased in positive polarity. It also decreases as “6V” output is increased in the negative polarity. The tail current is not constant, therefore, the value gm and amplification varies. This shows that the common mode rejection requires a constant tail current 1x. Applied input common-mode voltage produces amplifier input bias current. Modern current output digital-analog converters are made of differential outputs, so as to achieve high common mode rejection and reduce the distortion. Full scale output currents in the range of 2 mA to 20 mA are frequent. Conclusions: In general, the experimental results agree reasonably well with the expectations. This was used to show that the experiment was done correctly. The circuit for the common emitter amplifier is normally used in applications where a small voltage signal needs to be amplified to a large voltage signal. Since the amplifier cannot drive low resistance loads, it is usually cascaded with a buffer that can act as a driver. The following conclusions were also made. When low resistance is connected with the amplifier the voltage gain will decrease. The errors which appeared in this experiment are related to: 1. Thermal and time drift of the electronic and electric components used in the experiment. 2. Error in reading the results. 3. The resolution in the measurement devices.  References B. Chu-Kung, 2007. Compound semiconductor power heterojunction bipolar transistor technology, University of Illinois at Urbana-Champaign. S. W. Amos; Mike R James, 2000. Principles of transistor circuits: introduction to the design of amplifiers, receivers and digital circuits, Oxford: Newnes B. Duncan, A.M.I.O.A, 1996. High performance audio power amplifiers for music performance and reproduction, Oxford; Boston: Newnes Read More