The Use of Tuned Amplifier - Lab Report Example

THE TUNER CIRCUIT (LAB REPORT) The tuned amplifier is an amplifier for amplifying narrow band of frequencies or specific frequency. The tuned amplifiers are utilized in the amplification of radio or high frequencies; since their radio frequencies are usually single and the tuned circuit allow their selection with efficient amplification. The circuit was implemented via a diagram set up, then the function generator was connected and the input signal was set at 100 mV peak to peak at 5 kHz. One channel of the oscilloscope was used in setting voltage; the other channel was connected to the output. The frequency was increased at the input and observed at the output; and then sketched. From calculations, the resonant frequency was 1.56 X 105 Hz, the circuit’s Q-factor for the tank circuit was 888.7 and the bandwidth was 176.1 Hz. Moreover, from the graphs, at the input frequency equal to resonant frequency, i.e. fr =fin , the voltage across the output resistor was maximum hence maximum voltage observed. Introduction An Amplifier that amplifies narrow band of frequencies or specific frequency is known as the tuned amplifier. The tuned amplifiers are utilized in the amplification of radio or high frequencies; this is because the radio frequencies are usually single and the tuned circuit allows their selection with efficient amplification (Godse and Bakshi, 2010). Tuned amplifiers, however, are not suitable when amplifying the audio frequencies since they are made of mixture of frequencies ranging from 20 Hz to 20 kHz; hence, they not single. The tuned amplifiers are utilized in television and radio circuits where they are also known as upon to handle radio frequencies (Kal, 2013). Figure 1 shows a simplified tuned amplifier circuit. In the circuit, instead of having a load resistor, a parallel tuned circuit is implemented in the collector. The tuned circuit’s impedance is strongly dependent on frequency. The circuit avails very high or elevated impedance at resonant frequency; consequently, there are minimum impedance at all remaining or other frequencies. In case the signal possesses same frequency as the LC circuit’s resonant frequency, the large amplification emanates due to LC circuit’s high impedance at this frequency. In case of signals with varied frequencies are availed at the input of the tuned amplifier, the circuit will select and then strongly amplify the signals of the resonant frequency and reject all others. Hence, such amplifiers are very vital in radio receivers in terms of signal selection from particular broadcasting station from a pool of signals of other frequencies available at the receiving aerial (Lee, 2011). Figure 1. Tuned amplifier (Rao, 2012) It is compulsory in the high frequency applications to amplify single frequencies and reject or ignore other frequencies available; hence, the tuned amplifiers are utilized. The amplifiers utilize tuned parallel circuit for the collector load and avails many advantages. There are small power losses since the tuned parallel circuit incorporates the reactive components C and L. Additionally; the general power loss in the circuit is quite low and therefore makes the tuned amplifiers highly efficient (Thyagarajan, Chelvi and Rangaswamy, 2009). The circuit has high selectivity property; therefore, it selects the wanted or desired frequencies for amplification from a number of frequencies. The circuit minimum collector supply voltage; since the parallel tuned circuit exhibits low resistance value. The circuit requires reduced collector supply voltage (Hood, 2013). Method Figure 2. Parallel tuned circuit practical set up Once the circuit in figure 2 was implemented, a function generator was connected and the input signal was set at 100 mV peak to peak at 5 kHz. One channel of the oscilloscope was used in setting voltage, the other channel was connected to the output; the output should not display any signal or little signal. The frequency was increased at the input and observed at the output; and then sketched. Calculations Figure 3. Parallel tuned circuit set up (Rao, 2012) The parallel tuned set is made of the inductor L and capacitor C as illustrated in figure 3. In practical conditions, there exists some resistance in the coil, represented by R. When an alternating voltage is input across the parallel circuit, there will be frequency oscillations of the applied alternating voltage. Consequently, if the applied voltage’s frequency is equal to the resonant or natural frequency of the LC circuit, electrical resonance occurs. In this condition, there exists high impendence in the tuned circuit while there is minimum line current. Hence the circuit draws enough energy emanating from the A.C. supply towards overcoming the losses via the resistance R (Rao, 2012). The circuit’s parallel resonance when containing the reactive elements, that is C and L, become the resonant when the tuned circuit power factor is at unity i.e. the supply current and applied voltages are in phase. Figure 3, which is a phasor diagram shows that IL, which is the coil current, having two rectangular components; the reactive component or the ILsinand the active component or the ILcos . The parallel circuit resonates when power factor is unity; hence, this is possible when total or net reactive components are equal to zero, as shown by the equation below: Ic- ILsin=0 The resonance of parallel is achieved through the change of supply frequency. At the resonant frequency, fr, Ic= ILsinwhen resonance occurs (Somanathan and Deepa, 2009). At parallel resonance, Ic= ILsin sin= XL/ZL ; IL=V/ZL ; Ic = V/Xc = · = = = + Therefore the resonant frequency or fr = , hence for the small coil resistance, R then the equation becomes: fr = in Hertz From figure 2, L=47mH; C=22 pF; and Manufacturer’s maximum R amount = 52 Ω (Uk.farnell.com, 2015) The resonant frequency, fr = = =1.56 X 105 Hz The Q-factor of the tank circuit XL = =·1.56 X 105 · = 4.62 X 104 Ω Q= = 888.7 The bandwidth of the amplifier BW==176.1 Hz Results and Discussion Figure 4. Output trace from the 65 kHz input signal From the graph in figure 4, the input frequency was equal to resonant frequency, i.e. fr =fin , therefore the parallel LC circuit avails very high impedance that acts as if an open. Since the output’s resistor avails the only path to the ground at the collector circuit, hence the entire alternating current collector current flow via the output’s resistor. Therefore the voltage across the output resistor is maximum; this is shown by maximum voltage gain in figure 1. Figure 5. Output signal trace from the 71 kHz input signal Figure 5 represent the scenario with input frequency greater value as compared to the resonant frequency. Since the input frequency is high, then the circuit is effectively capacitive in nature. When the input frequency is increased beyond the resonant frequency, then a point where XL – XC = RL is achieved, RL is the output resistor; this leads to voltage gain drop. Discussion The voltage peak is narrow from the figure 4, at this frequency, the circuit was experiencing high Q factor, since the quality Q is represented as: Q = . The high Q signifies that there is small value or little total series resistance; for the coil resistance or any other resistance. In this case, Q is represented by Q = , in which XL = 2pfoL, while fo is the resonant frequency. This resonance phenomena is vital in communication, for example in radio, since this increases the sensitivity of the reciver, in addition, it avails selectivity by enabling the signals of defined frequency to be magnifies in order to separate them from othe signals. From the experiment, it can be seen that, when the circuit is at resonance Xc = XL and the phase shift is zero; hence the circuit exhibits pure resistance. Whe the circuit was below the resonance, Xc was greater than XL, and the phase shift was negative hence the circuit was purely capacitive. When the same circuit operated at frequency greater than resonance frequency, the XL was greater than Xc while the was a positive phase shift and the overall circuit was inductive. Conclusion The high frequencies input signals to be amplified are fed to the amplifier’s input. The parallel tuned circuit’s resonant frequency is adjusted until it is equal to the input signal’s frequency by changing the values of capacitances. In these conditions, the parallel tuned circuit will avail very high impedance towards the signal frequency. Therefore a large output exhibited across the practice’s tuned circuit. References Godse, A. and Bakshi, U. (2010). Electronic circuits - II. Pune, India: Technical Publications. Hood, J. (2013). The art of linear electronics. Oxford [England]: Butterworth-Heinemann. Kal, S. (2013). Basic electronics. New Delhi: Prentice-Hall of India. Lee, J. (2011). Advanced Electrical and Electronics Engineering. Berlin, Heidelberg: Springer Berlin Heidelberg. Rao, B. (2012). Electronic circuit analysis. Chennai: Pearson. Somanathan Nair, B. and Deepa, S. (2009). Basic communication and information engineering. New Delhi: I.K. International Publish House. Thyagarajan, T., Chelvi, K. and Rangaswamy, T. (2009). Engineering basics. New Delhi, India: New Age International (P) Ltd. Uk.farnell.com, (2015). TOKO-47 MH INDUCTOR -Farnell. [online] Available at: http://uk.farnell.com/toko/10rb473k/inductor-47mh/dp/1193626 [Accessed 5 Apr. 2015]. Read More