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# The aim of this project is to design Essay

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The largest trimmer capacitor available is 65pF, and there has to be connected in parallel by a fixed capacitance of 330pF. 3. Signal Amplification The tuned circuit is connected to the collector of a common emitter amplifier as shown in figure 3. The base of T2 is connected to the output of the input stage. The gain of the common emitter amplifier is given by G i?? ZRC/ (re + R1//Zc) where ZRC is the impedance of the tuned circuit at resonance and the emitter resistance re = 25mV/Ic.

At signal frequency, capacitor C2 bypasses resistor R1 and its impedance is given by Zc2 i??re/10. So, the gain, G i?? 10 (ZRC/ re). Resistor R1 limits the quiescent emitter current to make it smaller than 1mA. Under quiescent conditions, the emitter voltage of T2, VE = (1. 55-0. 7) = 0. 85V. Thus, R1 is selected to be 2. 8k? for a quiescent emitter current of i?? 0. 4mA. C2 is chosen to have a value of 0. 1 i?? F so that Zc2 = 8.

23 i?? re/10 at the signal frequency. 4. Demodulation The demodulator circuit is to recover the low frequency audio signal from the high frequency carried. This is done by filtering out the high frequency carrier signal.

Therefore, a low-pass filter is used. The input, Vout2 i?? 6V d. c. to ensure that the whole signal is positive. The emitter voltage of T3 will then be i?? 6-0. 7 = 5. 3V. To ensure an emitter current of 1mA, R4 is chosen to be 5. 3k?. C3 has to be chosen such that bandwidth < 1/RC < f0. The audio signal will have a frequency of i?? 4 kHz – 8 KHz. From the transfer function of the filter, H (? ) = R4/ (1+j? R4C3), C3 can be determined for the desired cut-off frequency ? = 1/R4C3. A capacitor of 10nF is used to give a cut-off frequency of i?? 15KHz. 5. Power amplification and audio output.

The demodulated audio signal needs to be amplified to bring it up to the level required by the earphone (with impedance of 8k? ). The voltage leaving the demodulator should be i?? 0. 1Vp-p. For an adequate volume at the earphones, an output of at least 0. 2Vp-p is required. An opamp with a gain of 4 will be used to carry out the amplification. The mid-band gain of a non-inverting amplifier as shown in figure 5 is (1+R6/R5). To attain a gain of 4, R6 = 4k? and R5 = 1k?. The capacitors C4 and C5 can be used to set the lower and upper cut off frequencies.

The audio signal is approximately between 150Hz and 4.5 kHz. These cut-off frequencies are to ensure that an audio signal with limited distortion is received at the headphones. Lowest frequency, ? = 1/R5C4 = 2? (150), therefore C4 = 1 i?? F. Highest frequency, ? = 1/R6C5 = 2? (45000), therefore C5 = 8. 8nF. A decoupling capacitor C6 will be place at the output of the opamp in series with the earphone to remove the DC component of the audio signal. The impedance of the earphone (8? ) should be significantly larger than the impedance of C6 at the audio frequency so that the strength of the amplified signal is not reduced.

A 220 i??F capacitor will be used for this purpose, as it has an impedance of i?? 1? at and audio frequency of 4 kHz. 6. Power Supply Rails The circuit used to bias the voltage is shown in figure 6. The reverse zener current has to be at least 5mA in order for the diode to supply a constant voltage drop (reverse breakdown voltage). In this circuit, the load current contributes to the current across the zener, driving it into breakdown. A voltage source of 4V is required to supply the cascaded amplifier in figure 3 and a source of 6V is required to supply the tuned circuit in figure 2.

For Vout = 6V, R7 = 1. 2k? , and for Is = 5mA and a zener with a reverse breakdown voltage of 3V is required. For Vout = 4V, R7 = 800? and Is = 5mA, a zener with a reverse breakdown voltage of 5V is required. C7 acts as a decoupling capacitor which has lower impedance at the signal frequency and at noise frequency (50 KHz) than the circuit impedance (i?? 1k? ). C7 is chosen to be 100nF. Conclusion With a budget limit, a radio that costs was designed, and does not exceed the budget of i?? 6. 30.

However the actual component values may differ from the calculated and amendments will have to be made during the testing and building stage for the radio to clearly receive the desired radio station (Virgin AM at 1. 215 MHz). Please refer to the appendix for the complete radio design. References: 1. Microelectronics Circuits 4th Edition, Sedra & Smith 2. 1st year Electronics Lab Booklet, Spring Term 2004 Appendix: The Final Circuit Design Show preview only The above preview is unformatted text This student written piece of work is one of many that can be found in our GCSE Waves section.

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