Tag: Transistor circuits

An ideal transistor would show 0% distortion in amplifying a signal. Its gain would extend to all frequencies. It would control hundreds of amperes of current, at hundreds of degrees C. In practice, available devices show distortion. Amplification is limited at the high frequency end of the spectrum. Real parts only handle tens of amperes…

Like all electrical and electronic components, transistors are limited in the amount of voltage and current each one can handle without sustaining damage. Since transistors are more complex than some of the other components you’re used to seeing at this point, these tend to have more kinds of ratings. What follows is an itemized description…

Bipolar Junction Transistor or BJT Current Mirror An often-used circuit applying the bipolar junction transistor is the so-called current mirror, which serves as a simple current regulator, supplying nearly constant current to a load over a wide range of load resistances. We know that in a transistor operating in its active mode, the collector current…

Input impedance varies considerably with the circuit configuration shown in Figure below. It also varies with biasing. Not considered here, the input impedance is complex and varies with frequency. For the common-emitter and common-collector, it is base resistance times β. The base resistance can be both internal and external to the transistor.  For the common-collector:…

If some percentage of an amplifier’s output signal is connected to the input, so that the amplifier amplifies part of its output signal, we have what is known as feedback. Feedback Categories Feedback comes in two varieties: positive (also called regenerative), and negative (also called degenerative). Positive feedback Reinforces the direction of an amplifier’s output…

To overcome the challenge of creating necessary DC bias voltage for an amplifier’s input signal without resorting to the insertion of a battery in series with the AC signal source, we used a voltage divider connected across the DC power source. To make this work in conjunction with an AC input signal, we “coupled” the…

Although transistor switching circuits operate without bias, it is unusual for analog circuits to operate without bias. One of the few examples is “TR One, one transistor radio” TR One, Ch 9 with an amplified AM (amplitude modulation) detector. Note the lack of a bias resistor at the base in that circuit. In this section,…

In the common-emitter section of this chapter, we saw a SPICE analysis where the output waveform resembled a half-wave rectified shape: only half of the input waveform was reproduced, with the other half being completely cut off. Since our purpose at that time was to reproduce the entire waveshape, this constituted a problem. The solution…

While the C-B (common-base) amplifier is known for wider bandwidth than the C-E (common-emitter) configuration, the low input impedance (10s of Ω) of C-B is a limitation for many applications. The solution is to precede the C-B stage by a low gain C-E stage which has moderately high input impedance (kΩs). The stages are in…

The final transistor amplifier configuration (Figure below) we need to study is the common-base amplifiers. This configuration is more complex than the other two and is less common due to its strange operating characteristics. Common-base amplifier Why is it Called a Common-base Amplifier? It is called the common-base configuration because (DC power source aside), the…

Our next transistor configuration to study is a bit simpler for gain calculations. Called the common-collector configuration, its schematic diagram is shown in the figure below. Common collector amplifier has collector common to both input and output. It is called the common-collector configuration because (ignoring the power supply battery) both the signal source and the…

At the beginning of this chapter, it illustrates how transistors could be used as switches, operating in either their “saturation” or “cutoff” modes. In the last section, we saw how transistors behave within their “active” modes, between the far limits of saturation and cutoff. Because transistors are able to control current in an analog fashion,…

When a transistor is in the fully-off state (like an open switch), it is said to be cutoff. Conversely, when it is fully conductive between emitter and collector (passing as much current through the collector as the collector power supply and load will allow), it is said to be saturated. These are the two modes…

Bipolar transistors are constructed of a three-layer semiconductor “sandwich” either PNP or NPN. As such, transistors register as two diodes connected back-to-back when tested with a multimeter’s “resistance” or “diode check” function as illustrated in the figure below. Low resistance readings on the base with the black negative (-) leads correspond to an N-type material…

Bipolar junction transistors (Also known as BJTs) can be used as an amplifier, filter, rectifier, oscillator, or even a switch, which we cover an example in the first section. The transistor will operate as an amplifier or other linear circuit if the transistor is biased into the linear region. The transistor can be used as…