Why would you not routinely use a potentiometer for this purpose? Procedure 5 (2

Question

Why would you not routinely use a potentiometer for this purpose?

Procedure 5 (25 points) Voltage Divider Design Figure P5-1 shows a transistor amplifier circuit, the basis for any number of electronic products like cell phones, radios and stereos. An important part of designing this RI circuit is biasing the transistor, meaning to set the steady state DC voltage of the transistor base, vr. This is done with a voltage divider 9 TV consisting ofresistons RI and R2. At first glance, RI and R2 are not in a voltage divider configuration. There are two additional connections to node b. which connects the resistors. However, by using some knowledge about the circuit and the conditions of interest, both of these connections will prove to be approximately Figure P5-1 Common emitter transistor amplifier. open circuits. The first connection is a capacitor between node b and the input v In steady state DC, capacitors are open circuits (see procedure oi in this lab, for example) So for purposes of biasing the transistor, the connection to v is an open circuit The second connection is the transistor base. When you learn about transistors, you will discover that the base current, ih, is "small". When someone says "small like that, what is really meant is i is approximately zero. Zero current is an open circuit. So you see that both of the extra connections can be treated as open circuits when computing the bias voltage, and RI and R2 are a voltage divider for this purpose. This is a good example of how the ideal configurations we study in basic circuit theory are "hidden" in real circuits, and how knowledge of the component behavior can be used to make approximations that reduce the real circuit to a simpler one. Alarge part ofengineering knowledge is learning when approximations are valid, and when they are not. a. (15 points) Design a voltage divider using the set ofresistons supplied with the lab kit to obtain a value of v 4.85 V. Get as close to this value as you can. Ensure that the current through the voltage divider is greater than 0.1 mA (this keeps the "ia is small" approximation true) Ensure that no resistor is overloaded (dissipates power greater than its power rating). Record your design (draw circuit and label values and voltage and power calculations. b. (5 points) Implement your design Gust the voltage divider part) on your breadboard. Measure and record the bias voltage you obtained. Show this to your TA, including measuring the desired voltage. c. (5 points) Implement the voltage divider with a 10Kepotentiometer. Adjust the potent ter to get the voltage vb as close as possible to 4.85 V. Sketch the circuit and record the value of vb you achieved.Mhy would you not routinely use a potentiometer for this purpose Hint: Check the prices for resistors and potentiometers on the EE Stores web page. What else costs money in step?

Explanation / Answer

Potentiometer can not be used routinely for this purpose due to the following reasons

1. Potentiometer is very costly as compared to routinely used resistors.

Price of 10 K.Ohm Potentiometer = $ 20.00

Where as Resistors kit (365 pieces) costs = $ 7.50

2. As well Potentiometer is very sensitive device can be damaged easily so resistors are a better option.

3. Measurement with the help of Potentiometer is a time taking process.

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