66.6.2. I-V Characteristic Curve

This is a recapitulation or recalling whatever we know about CE amplifier and not only we will be talking about CE amplifier. But basic operation of the CE amplifier just to see that, what is it is limitation of the voltage gain.

In fact, if you recall that this is this is the main amplifying transistor and it is at the input we do have the signal we are feeding along with the along with the DC component. So, that the transistor it is in active region of operation. In addition to that we also have the R connected to the collector to the supply voltage V and the connection of this R it is C such that the transistor here it is in active region of operation. So, this R it is having dual role to play, first of all it provides appropriate region of operation. And the second one it is it also converts the current into voltage, because primarily at the output we observe the signal in the form of voltage V . So, by applying a voltage at the input port namely at the base it is given here which is having a DC voltage along with the signal. We are changing the collector current with respect to it is quiescent current and that variation or change or the signal part it is getting converted from current to voltage by this resistor R .

The common emitter (CE) amplifier is a type of transistor amplifier that amplifies input signals. It uses a transistor as the main amplifying element, where a signal (with a DC component) is fed into the base terminal. The purpose of the input DC voltage is to keep the transistor in its active region, allowing it to properly amplify signals. A resistor, labeled as R, is connected to the collector of the transistor and helps determine two key aspects of the amplifier's operation: it ensures the transistor operates in the active region and converts the output current into an output voltage. When the input signal changes the base current, this causes a change in the collector current, which in turn causes a voltage drop across the resistor, allowing us to read a voltage output from the amplifier.

Think of the common emitter amplifier as a water faucet. The input flow of water (DC voltage) keeps the faucet open (active region), allowing water to flow freely. The resistor at the output represents a bucket that collects the water (voltage output). When you adjust the faucet (change base voltage), more or less water flows into the bucket, creating either a higher or lower water level (output voltage).

Now, today we are planning to cover it is under this active load amplifiers, we do have to start with we do have motivation of using this active load. Then, from that we will we will be talking about basic operation of amplifier having active load and their corresponding circuit analysis including, small signal model and then finding the gain or maybe intuitively explaining the gain and so and so.

The discussion is moving towards the limitations of the voltage gain in common emitter amplifiers when using a passive load. It has been noted that the gain of a CE amplifier is primarily limited by the voltage drop across the resistor. As such, there is motivation to replace this passive component with an active load to enhance gain. By using an active load, which uses a transistor instead of a passive resistor, the voltage gain can be significantly improved. The circuit analysis will include finding the gain and discussing small-signal models to demonstrate the improvements.

Imagine trying to fill a glass with water using a pipe that has a narrow opening (passive load); it's slow, and water pressure drops significantly. By switching to a thicker, flexible hose (active load), water can flow much faster, effectively increasing the output. The 'active load' lends itself to enhancing the performance where the passive load struggles.

So, pictorially if you see the output for characteristic, namely if we sketch the I versus V . So, you may recall for a given value of current at the base the corresponding collector current it is having I-V characteristic like this. So, in the active region the current I it is almost independent of V . But then if you go very low then of course, the device enters into saturation region and then there is a significant or I should say sort dependency of the collector current on V or in this case incidentally that is V .

In plotting the I-V characteristic curve for the common emitter amplifier, we examine how the output collector current varies with the output voltage across a load resistor. In the active region of operation, the collector current is relatively constant with variations in output voltage. However, when the voltage drops too low, the transistor moves into saturation, altering this relationship significantly due to the dependence of the collector current on the output voltage.

Think of a car accelerating at a steady speed (active region) regardless of the speed bumps (output voltage). If the bumps are too high (low output voltage), the car struggles to maintain speed or even stalls (saturation).

Now, then if we consider the load line characteristic, as you have discussed load line characteristic it is given by essentially I-V characteristic of this R . And we have discussed that how we obtain this load line characteristic; namely if you plot the voltage the current through this resistance R with respect to it is it is voltage across it is V . Actually this load line characteristic is linear.

The load line characteristic is an important aspect in analyzing the performance of the CE amplifier. It is derived from the I-V characteristics of the resistor (R) and represents the maximum achievable current versus voltage for the load. This load line is linear, and plotting it on the same diagram as the transistor's I-V characteristics allows us to identify points of operation and gain. The intersection of the load line with the transistor's I-V curve reveals the quiescent point, which is critical for amplifier stability and performance.

Imagine plotting a route on a map (I-V characteristic) and using a straight line (load line) to represent your maximum speed. The point where your route meets the straight line shows how fast you can drive to reach your destination efficiently without getting slowed down.

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