The main aim of this assignment is to design a pre amplifier circuit with an NPN transistor to be used in a simple public address (PA) system. The pre amplifier is fed from a microphone that produces an average output voltage of 10 mV rms. The amplifier is to operate over a frequency range of 300 Hz to 5 kHz and should have an adjustable volume control. The expected gain of the amplifier is 100. First we are going to design an amplifier for given specifications, model the operation of the circuit using h-parameter and r-parameter model, use computer aided design software to analyze the circuit performance and demonstrate the working of the circuit by hardware implementation. Then, we will plot the frequency response of the circuit and analyze the effect of the emitter bypass capacitor. Finally we will compare the mid-band gain, bandwidth and lower cutoff frequency obtained from the simulation result and the hardware implementation with the designed values.
Bipolar Junction Transistor (BJT) is a three terminal device with three regions (Emitter, Base and Collector) and two PN junctions (Emitter-Base junction and Base-Collector junction). Since there are two junctions that means there are four possible ways of biasing a transistor. If both junctions are forward biased then the transistor will operate in the saturation region. If both junctions are reverse biased then the transistor will operate in the cut of region. These two conditions of operation are used when the transistor is needed to work as a switch. To use a transistor as an amplifier, the emitter base junction should be forward biased and the collector base junction should be reverse biased. Amplifier is an electronic circuit that can amplify signals applied to its input terminal. If an AC signal is given to a transistor amplifier it will produce an AC base current. This AC base current will produce a much larger AC collector current since IC=βIB. The AC collector current produces an AC voltage across the load resistor RL, thus producing an amplified, but inverted, reproduction of the AC input voltage in the active region of operations. DC load line is a sloping straight line connecting all the operating points of a transistor biasing drawn on the output characteristics of the transistor and the intersection point gives the Quiescent point (Q-point). A prober Q-point should be in the middle of the DC load line. Selecting a good Q-point prevents the transistor from going into the cutoff or the saturation region and gives more stability. A fixed bias (i.e. base bias) circuit or a voltage divider bias circuit can be used for this assignment but a voltage divider circuit is more efficient. The main disadvantage in a fixed bias circuit is that βac depends on temperature, which means βac is not stable. And when βac changes, IC will change(IC=βIB) and VCE will change. The changes in IC and VCE make the Q-point unstable. Whereas in voltage divider bias circuit, IC is independent of βac and hence the Q-point is more stable. Voltage divider bias is widely used because reasonably good stability reached with a single power supply.
The problem is to design and fabricate a pre amplifier circuit with an NPN transistor to be used in a simple public address (PA) system. The input of the pre amplifier circuit is taken from a microphone that produces an average output voltage of 10 mV rms. The amplifier is to operate over a frequency range of 300 Hz to 5 kHz. Also, it should have an adjustable volume control. The expected voltage gain of the amplifier is 100. Design Specifications:
Voltage gain = 100
Lower cut off frequency = 300Hz
Vin = 10mV (rms)
RL = 10kΩ
DC power supply = 10V to 15V
Type of transistor – NPN
We will begin our assignment by selecting a suitable transistor. Then we will decide on a DC voltage supply and assume a prober Q-point (IC, VCE) to carry out the design. We will start the...