We call R L the loaded collector resistance this will be the parallel of R3 and R i. Now we can compute the overall gain of each stage loaded by the previous one. Simulating the graph analysis and maximizing it we can find that input resistance is 13.9kΩ. This allows us to plot the result of a formula based on probes on the circuit, in this case making use of the voltage and current probes to plot resistance. The trick to getting the the input resistance plotted on the graph is to use what's called a trace expression. We can arrange a schematic file like thisĪ demo copy of Proteus is required to open the. We can use DC Transfer Curve Analysis (available in Proteus and other SPICE tools) to help us to characterize the 2N3904 in order to find the working point in steady state condition. For our design we will use the 2N3904 in the linear region as an amplifier. The 2N3904 is an epitaxial planar NPN, general purpose transistor for small signals and switching application. We're going to use the popular 2N3904 NPN BJT. We will supply this amplifier with a 12Vdc power supply. We need an amplifier with at total Gain greater than 46dB, (Gain > 200), and a frequency response from less than 10Hz up to 1MHz -3dB. In order to get the input impedance high enough we will set a collector current I C as low as possible 1mA will be sufficient General Specification This is the effect of the input impedance of the next stage against the gain of the previous one. However, the value of each stage gain must consider the loading effect. The total gain is calculated as the product of each gain of any individual stage: Gain = A1 x A2 x A3 x A4
This approach is fine if, as in this case, we don’t require the DC amplifier response. In other words we are connecting each stage by using a capacitor in series between the output of the previous stage with the input of the next one. The amplifier we are going to design is a simple cascade connection where four identical stages are connected by RC coupling. There are several advantages with multistage amplifiers, notably achieving high gain with good frequency response and low level distortion. This article shows a design process for a simple four stage BJT Amplifier based on the popular silicon 2N3904 NPN transistor.
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