Voltage current relationship in a transistor amplifier

Transistor Common Emitter Amplifier | Electronics Notes

voltage current relationship in a transistor amplifier

A transistor amplifier is a current control device. The diagram is a transistor amplifier circuit. The it will graph the relationship of current and voltage gain. Active Region – the transistor operates as an amplifier and Ic = β*Ib; Saturation Common Emitter Configuration – has both Current and Voltage Gain. Since the electrical relationship between these three currents, Ib, Ic and Ie is determined. Voltage, Current, Resistance, and Ohm's Law – An introduction to the fundamentals Electric Power – One of the transistors main applications is amplifying . linear relationship between voltage and current, transistors are non -linear devices.

When output voltage VCE is at zero volts and emitter-base junction is forward biased by input voltage VBEthe emitter-base junction acts like a normal p-n junction diode. So the input characteristics of the CE configuration is same as the characteristics of a normal pn junction diode. The cut in voltage of a silicon transistor is 0.

In our case, it is a silicon transistor.

Transistor Common Emitter Amplifier

So from the above graph, we can see that after 0. In common emitter CE configuration, the input current IB is produced in the base region which is lightly doped and has small width. So the base region produces only a small input current IB. On the other hand, in common base CB configuration, the input current IE is produced in the emitter region which is heavily doped and has large width.

So the emitter region produces a large input current IE. Therefore, the input current IB produced in the common emitter CE configuration is small as compared to the common base CB configuration. Due to forward bias, the emitter-base junction acts as a forward biased diode and due to reverse bias, the collector-base junction acts as a reverse biased diode. Therefore, the width of the depletion region at the emitter-base junction is very small whereas the width of the depletion region at the collector-base junction is very large.

voltage current relationship in a transistor amplifier

If the output voltage VCE applied to the collector-base junction is further increased, the depletion region width further increases. The base region is lightly doped as compared to the collector region. So the depletion region penetrates more into the base region and less into the collector region. As a result, the width of the base region decreases which in turn reduces the input current IB produced in the base region.

From the above characteristics, we can see that for higher fixed values of output voltage VCE, the curve shifts to the right side.

Basic BJT Amplifier Configurations

This is because for higher fixed values of output voltage, the cut in voltage is increased above 0. Therefore, to overcome this cut in voltage, more input voltage VBE is needed than previous case. Those layers are capable of carrying a current.

Transistors are devices that can amplify a signal in a circuit. It is normally made of germanium or silicon layers.

voltage current relationship in a transistor amplifier

A transistor requires very little current to operate so, releases very little heat. However, heat also depends upon it configuration. It is used in different things, such as an amplifier, oscillator, rectifier, switch, etc. A signal is just a general term used to refer to any particular current, voltage, or power in a circuit.

Amplification is a ratio between two values which does not imply that the output value is greater than the input value. Transistor amplifying current Transistors are normally used as amplifiers. Some transistor circuits are current amplifiers, with a small load resistance, other circuits are designed for voltage amplification and have a high load resistance and others amplify power.

Because the transistor is a current-regulating device, and because meter movement indications are based on the current through the movement coil, meter indication in this circuit should depend only on the current from the solar cell, not on the amount of voltage provided by the battery. This means the accuracy of the circuit will be independent of battery condition, a significant feature!

All that is required of the battery is a certain minimum voltage and current output ability to drive the meter full-scale.

Voltage Output due to C urrent oad Resistor Another way in which the common-emitter configuration may be used is to produce an output voltage derived from the input signal, rather than a specific output current.

With the solar cell darkened no currentthe transistor will be in cutoff mode and behave as an open switch between collector and emitter.

voltage current relationship in a transistor amplifier

This will produce maximum voltage drop between collector and emitter for maximum Voutput, equal to the full voltage of the battery. At full power maximum light exposurethe solar cell will drive the transistor into saturation mode, making it behave like a closed switch between collector and emitter.

The result will be minimum voltage drop between collector and emitter, or almost zero output voltage. In actuality, a saturated transistor can never achieve zero voltage drop between collector and emitter because of the two PN junctions through which collector current must travel. For light exposure levels somewhere between zero and maximum solar cell output, the transistor will be in its active mode, and the output voltage will be somewhere between zero and full battery voltage.

An important quality to note here about the common-emitter configuration is that the output voltage is inverted with respect to the input signal. That is, the output voltage decreases as the input signal increases.

How does a transistor amplify current? - Physics - Metropolia Confluence

For this reason, the common-emitter amplifier configuration is referred to as an inverting amplifier. A quick SPICE simulation Figure below of the circuit in Figure below will verify our qualitative conclusions about this amplifier circuit. At the beginning of the simulation in Figure above where the current source solar cell is outputting zero current, the transistor is in cutoff mode and the full 15 volts from the battery is shown at the amplifier output between nodes 2 and 0. Notice how the output voltage trace on the graph is perfectly linear 1 volt steps from 15 volts to 1 volt until the point of saturation, where it never quite reaches zero.

This is the effect mentioned earlier, where a saturated transistor can never achieve exactly zero voltage drop between collector and emitter due to internal junction effects.

What we do see is a sharp output voltage decrease from 1 volt to 0. The lowest the output voltage ever gets in this simulation is 0.

The Common-emitter Amplifier

In the solar cell light meter example, we were interested in amplifying the DC output of the solar cell to drive a DC meter movement, or to produce a DC output voltage. However, this is not the only way in which a transistor may be employed as an amplifier. Often an AC amplifier for amplifying alternating current and voltage signals is desired.

One common application of this is in audio electronics radios, televisions, and public-address systems. Earlier, we saw an example of the audio output of a tuning fork activating a transistor switch.

Transistor switch activated by audio. All we cared about here was turning the lamp on with a sound signal from the microphone, and this arrangement sufficed for that purpose.

voltage current relationship in a transistor amplifier

But now we want to actually reproduce the AC signal and drive a speaker. Common emitter amplifier drives speaker with audio frequency signal.

The simulation plots Figure above both the input voltage an AC signal of 1. What we see here is a full AC sine wave alternating in both positive and negative directions, and a half-wave output current waveform that only pulses in one direction.

If we were actually driving a speaker with this waveform, the sound produced would be horribly distorted. The answer to this question is found by close inspection of the transistor diode current source model in Figure below. The model shows that base current flow in on direction. Collector current is controlled, or regulated, through the constant-current mechanism according to the pace set by the current through the base-emitter diode.