However, there is one thing that is evident that cannot happen in a real device and that is having I XY be something other than zero when the voltage V XY is zero. Second, the current I XY is equal to 1mA per volt applied to the Z terminal with respect to the X terminal (figure 8.1.1). For the current controlled case we also reverse the direction of the control current I Z which now flows out of terminal Z.ДЇigure 8.1.3 Ideal Voltage (or Current) controlled current source characteristicsДЇrom these characteristic curves we can learn the following first the current is indeed independent of the voltage across the X and Y terminals. The same can be said for the voltage seen at terminal Z with respect to terminal Y and voltage VZY is a negative number. Because direction of the current is now reversed, we generally assume the voltage seen at Y is greater than terminal X and voltage VXY is a negative number. We can also describe complementary devices by reversing the direction of the currents such that the controlled current now flows out of terminal X and into terminal Y as shown in figure 8.1.2. This parameter of a transistor is called transconductance and gm is the common usage. The output current to control voltage relationship, expressed as amps/volt, is dimensionally a conductance and the letter g is most often used to represent conductance. Comparing this back to the current controlled device we can say that О± = 1 and ОІ is infinite. Also, since a voltage is the control signal in this case, we will assume that no current flows into (or out of) terminal Z. This voltage now needs to be referenced with respect to one of the two other terminals and we will use terminal Y for our purposes here. The voltage on terminal Z now controls the amount of current in terminals X and Y. The ratio of IX to IY which is always less than unity, is also a measure of the device gain and is most often represented by the Greek letter О± (alpha).ДЇor the voltage controlled device let's assume, as we did before, that the current flows into terminal X and out terminal Y. The ratio of IX to IZ is the gain of the device and the Greek letter ОІ (beta) is used to represent this gain. To make the device useful it would be desirable to have the control current IZ very small relative to the much larger controlled current IX. Conservation of charge tells us that the sum of the currents flowing into a box must equal the sum of the currents flowing out. In the case of a current controlled device, let's assume that the control current IZ, flows into terminal Z and back out terminal Y. The same can be said for the voltage seen at terminal Z with respect to terminal Y and voltage VZY is a positive number. Because current flows into terminal X, we generally assume the voltage seen at X is greater than terminal Y and voltage VXY is a positive number. To describe the function of this block we first need to define the terminal currents IX, IY and IZ, and the terminal voltages VXY and VZY as shown in the figure. The third terminal, Z, is the control terminal. Let's also assume that the controlled current flows into terminal X and back out terminal Y. It has three terminals we will call them X, Y and Z for the moment. It is sometimes incorrectly attributed to a contraction of transresistance.ДЄ simple and general form of such a device is shown in figure 8.1.1. The first type of transistor successfully demonstrated was a current-controlled device.ДЄs a side note: The origin of the term transistor is a contraction of вЂњtransconductance varistorвЂќ, as Devices working on the principle of one current controlling another current are known as current-controlled devices. Devices utilizing a voltage as the controlling signal are, not surprisingly, called voltage-controlled devices. One type of active device uses a voltage to control the current while another type of active devices uses another current to be the controlling signal. All active devices control the flow of current through them.

For a circuit to be called electronic, it must contain at least one active device. An active device is any type of component with the ability to electrically control the flow of current (controlling one electric signal with another electric signal).

Transistor