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Because we built our own power supply, it is necessary to watch for current limitations of the transformer and IC regulator. As such, unnecessary current drawn from the power supply should be avoided. In particular, large base bias resistors were used because they limit the current sunk by using the divider, and the resistors are only needed to provide a reference voltage and a DC path for the base of the transistor.

All transistors used were 2N3904, except those in the power amplifier. All high frequency coupling capacitors were 0.1uF ceramic capacitors.

The Radio Frequency Amplifier circuit is as shown in Figure 1 along with the netlist for simulation. Figure 2 shows the frequency response of the amplifier. Since AM has a range of 0.54 MHz to 1.7 MHz, the bandwidth of the Amplifier was chosen to be within this region, as indicated on the frequency response curve.

The tank�s component values were chosen based on bandwidth and center frequency requirements:

Desired Bandwidth: B=1.3MHz

Center Frequency: w_o=1.1MHz

Given these requirements, it is found that values of L=22uH, C=1nF, and R=100Ohm are acceptable, yielding w_o=1.07MHz, and B=1.6MHz. These values are quite closely matched by experimentally determined values of w_o=1MHz and B=1.5MHz. As well, the center frequency gain of the RF amp is simulated at about 11.5dB. It was found experimentally that this gain was about 8dB.

The variable capacitor across the antenna coil has a range of 10 to 100pF. In addition to this capacitance, it was found that a fixed parallel capacitance of 50pF is suitable to maintain approximately 455kHz difference between the oscillator and RF signals.

The Mixer circuit is as shown in Figure 3 accompanied with the netlist used in simulation. The mixer is shown with two inputs representing the oscillator and RF Amplifier inputs. The simulation in Figure 4 shows the output waveform, which clearly describes an amplitude-modulated signal based on two input sine waves at 1MHz and 1.2MHz.

The Mixer was implemented using a two-quadrant current multiplier. A DC current of 1mA was chosen as the reference current to be drawn through the diff-amp pair. Consequently, the diff-amp has a single-ended gain of about �50V/V (verified almost exactly in experiment). The mixer inputs are both fed into the circuit using common-collector amplifiers. This was done for a few reasons:

• A relatively high input impedance is seen by earlier stages (i.e. oscillator and RF amplifier);
• The DC bias of the current mirror in the mixer is set quite conveniently by adjusting the ratio of the base divider resistors in the leftmost common-collector amplifier; and
• The differential input of the diff-amp pair needs to be biased with positive voltage (so as not to turn off the current mirror), and yet carry an AC signal. This is done by using two emitter resistors in series in the rightmost common-collector amp, and coupling the differential voltage across the upper resistor through to either base of the diff-pair. On Figure 3, nodes 1 and 2 are coupled in to either base of the diff-pair.

The output modulated signal is caused by a sinusoidal current being drawn through the emitters of the diff-pair. The ratio of this sinusoidal current to the input voltage from the oscillator is calculated as 22*10^-3.

The Oscillator as is shown in Figure 5 along with the netlist used for simulation. The Colpitts design was used to implement the oscillator. Our oscillator created a healthy signal, having peak to peak amplitude of 3V (about 20% of the supply voltage). It has lower and upper frequency limits of 0.5 MHz and 1.8 MHz, which are depicted in Figures 6 & 7 respectively. This was found to be the actual range in experiment also. The oscillator�s center frequency is given by:

Given that variable capacitors come in common values (a range of 5 to 100pF was found), a suitable value of L=1mH is determined for the required oscillator frequency range. Then C1 is fixed at 1nF while C2 is the variable capacitor.

The Intermediate frequency Amplifier is shown in Figure 8 along with the netlist that was used for simulation. Figure 9 shows the frequency response of the amplifier. It was desired that the center frequency be at 455 kHz and the bandwidth of the amplifier be enough to pass a radio station signal through. Hence the upper and lower 3dB frequencies are 460 kHz and 450 kHz respectively. Using a similar circuit and analysis as for the RF Amplifier, values of L=5uH, R=100Ohm, and C=24.3nF are found in simulation and experiment to yield the required bandwidth and center frequency. In simulation, the center frequency gain was found to be 26dB, while experimental results yielded about 25dB gain.

The peak detector circuit is shown in Figure 10(a) along with the netlist that was used for simulation. A sample input and corresponding output waveform is shown in Figure 11. The RC constant was chosen to be able to rectify a 1kHz envelope best, with minimal ripple. This would achieve a good output sound as the audio frequencies that the audio stage would pass are within that range. A Germanium Diode was used because it has a lower forward bias voltage; our diode had approximately 0.3V forward voltage. This is in contrast to the much higher forward bias voltage in other standard diodes. A lower forward voltage is needed to ensure that as much of the mixed signal envelope is rectified and passed to the audio stage as possible.

The audio stage is as shown in Figure 10(b) along with the netlist that was used for simulation. Figure 12 shows a sine wave input to the power amp, and the subsequent output to the speaker load. The pre-amp consists of two common-emitter amplifiers surrounded by common-collector amplifiers. Collector resistors in the common-emitter amps were chosen as large as possible for maximum gain, and emitter resistors were chosen for maximum collector current. This yields a gain of approximately 29dB for the entire pre-amp. The Power Transistors used were MJ2955 (PNP), and 2N3055 (NPN). The values of Beta used in the simulation were 40 and 50 respectively (as found on the spec sheet for these transistors given the expected output current).

The power amp meets the specified 10W RMS peak value and can exceed it, which the speaker that was used can handle.

The power supply is shown in Figure 13 along with the netlist that was used in simulation. The circuit was chosen to have a single output voltage of 15V. This was achieved by using a 4:1 transformer coupled with a 600V/4A rated bridge rectifier, and a 15V IC regulator. Figure 14 shows the output ripple of the power supply before the IC regulator. This corresponds to a total rectified supply voltage of 42V +/- 1%.

The individual components of the radio were simulated and tested successfully. When the radio was put together it was found that all the parts performed as expected and we were able to tune several radio stations. Although there was interference present in the output audio signal it is believed that this can be eliminated by using a soldered circuit on a circuit board rather than a breadboard, as is done in any commercial application. Also, additional filtering stages in the IF and RF amps would significantly improve signal-to-noise ratio.