Lab 5: RF Amplifiers
Lab 5 applied the amplifier concepts introduced in labs 2 and 3 to radio frequency signals. In this case, a two-stage amplifier was built by feeding a JFET common-source amplifier into a common-emitter amplifier. This amplifier was eventually used to boost the antenna signal before it reached the AM detector from lab 4.
The common source amplifier was built on a breadboard and driven with a 1230kHz, 10mVpp signal, amplitude modulated at 50% depth by a 1kHz signal. The input and output signals of the amplifier with a 1MΩ load resistance are shown below in Figure 1. The Q-point of the JFET and the voltage gain of the amplifier were measured with different load resistances of 1MΩ, 10kΩ, and 1kΩ
Figure 1: Input and output of common source amplifier with 1MΩ load resistance
An RF choke was added to the amplifier to increase its gain, and the amplifier was measured again with the 1MΩ, 10kΩ, and 1kΩ load resistances. The input and output signals are shown below in Figure 2, again with a 1MΩ load resistance, but with the RF choke in place. The output amplitude is now around 32.6mVpp compared to 11.5mVpp before adding the choke. The Q-point and gain measurements have been compiled into Table 1 below.
Figure 2: Input and output of common source amplifier with 1MΩ load resistance and added RF choke
Table 1: Q-point and gain measurements for various output resistances
After measuring the common-source amplifier, the common-emitter stage was added. The Q-point of the BJT was measured and found to be Q(1.88V, 4.29mA). An oscilloscope measurement of the input and output with a 10kΩ load resistance is shown below in Figure 3. The gain is significantly increased compared to the single stage CS amp. More gain measurements were made with additional load resistances to characterize the full two-stage amplifier. These measurements are organized in Table 2 below and graphed in Figure 4.
Figure 3: Input and output of two-stage CS-CE amplifier with 10kΩ load resistance
Table 2: CS-CE amplifier gain measurements for various load resistances
Figure 4: Graph of CS-CE amplifier gain vs load resistance
Having constructed the amplifier, its output was sent through the AM detector to ensure that the intelligence signal would be extracted from the amplified AM signal. Figure 5 below shows the unamplified AM input signal along with the amplified intelligence signal from the detector.
Figure 5: Unamplified input signal and intelligence signal output post-amplification