RF Amplifiers
A Radio Frequency (RF) Amplifier is used to boost the weak received signal and amplify it. Since AM detectors work best around a certain input amplitude, the RF amp enhances the signal received in order to achieve the best results throughout the circuit. It is important to note that the received signal is modulated, the RF amp boosts the signal in order for the detector to be able to detect it; if the received signal it not strong enough the detector will be unable to detect and extract the signal.
To be able to detect the weak signal, a high input impedance is needed, for that the common source (CS) amp is used. However, the common source amplifier has a low gain, thus, a CE amp is used to make a two stage amp boosting the gain significantly. The CE amp has been discussed previously, thus, CS amp is the main focus here, with different variations to how the circuit can be built.
Part 1: CS Amp
The Common Source (CS) amplifier, in contrast to the previously used transistors, is a MOSFET. The CS amp has a small gain but a high input resistance, which allows most of the weak signal to appear across all terminals. Figure 1 shows the simple CS amp used. The CS amp is similar to the CC amp in regards to the gain being proportional to the resistance seen from the output. In the CC amp, the gain increases when the Rc increases, similarly, the gain of the CS amp increases when Rd is increased. As demonstrated below in table 1.
Figure 1: CS amp.
Part 2: Radio Frequency Choke
It can be seen that the CS amp has a gain that is proportional to the resistance seen by the drain. The drain sees two resistances in parallel, RD and RL, in the AC circuit it is desired for the drain to have as much resistance and the parallel resistors lower the total resistance. An inductor can be introduced to eliminate the parallel resistance.
An inductor, ideally, appears as a short in a DC circuit and as an open in an AC circuit. By adding an inductor, the high frequency signal is choked or suppressed, allowing only the DC signal through. Radio frequency choke (RFC) is an inductor that has a value that is designed for radio frequencies and different frequencies, the higher the frequency, the higher the inductance needed.
By adding the inductor between the drain resistor and the drain the resistor cannot be seen by the drain and only the load resistor is accounted for, thus the higher the load resistance the higher the gain will be. Figure 4 shows the CS amp with RFC using a load resistance of 10kΩ. To observe how the load resistance affects the output figures #-# are presented to demonstrate the effect.
Table 1 shows a comparison between the properties of the CS amp with and without the RFC.
Figure 4: CS amp with RFC.
Table 1, shown below, shows a comparison between the CS amp and the CS amp with the RFC. It is worth noting that the Q-point for both circuits is the same, this is due to the inductor appearing as a short in the DC analysis, which sequentially does not affect the circuit in DC and the Q-point stays the same.
Table 1: Comparison between CS amp and CS amp with RFC.
Part 3: CS-CE amp
Since the CS amp can have a sufficient gain, but in case the gain is not enough a CE amp is added to the CS to make a two-stage CS-CE amp. As seen previously, the CE amp has a good gain, but it is important to keep in mind that the CS-CE output is going into the rest of the radio, it is critical to not make the gain higher than it needs to be, as it could cause clipping in the circuit if the amplitude of the input signal is excessive.
Below is a demonstration of the CE-CS amp circuit along with a comparison between the input and output signals shown for various load resistances. Additionally, the values have been tabulated, see table 2, and a graph is shown demonstrating the effect the load resistance has on this circuit.
Figure 6: CS-CE amp circuit.
Table 2: CS-CE amp gain.
Figure 10: CS-CE amp circuit.