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Use of Transistors as Voltage Amplifiers !

Use of Transistors as Voltage Amplifiers !
Use of Transistors as Voltage Amplifiers !
Use of Transistors as Voltage Amplifiers !
Use of Transistors as Voltage Amplifiers !
Use of Transistors as Voltage Amplifiers !
Use of Transistors as Voltage Amplifiers !
Use of Transistors as Voltage Amplifiers !
Use of Transistors as Voltage Amplifiers !
Use of Transistors as Voltage Amplifiers !
Use of Transistors as Voltage Amplifiers !
Use of Transistors as Voltage Amplifiers !

In order for the transistor to operate properly as an amplifier, the two PN junctions must be correctly biased with external voltages. The following figure shows the proper bias arrangement for both NPN and PNP transistors. When a transistor is biased, the BE (base-emitter) junction has a low resistance due to forward bias and the BC (base-collector) junction has a high resistance due to reverse bias.

 

Since IB is extremely small, ICIE. Bearing this in mind, we consider the transistor in the following figure with an AC input voltage Vin, applied in series with the VBB bias voltage and with an external resistor RC, connected in series with the VBB bias voltage. The AC equivalent circuit is shown to the right of the original circuit (DC sources ideally appear as a short to an AC voltage). The forward-biased base-emitter junction appears as a low resistance to the AC signal. This internal AC emitter resistance of the transistor is called re and it is an internal parameter of the transistor (lower-case parameters here indicate AC quantities). The AC emitter current is:

 

And since ICIE , the output voltage developed across RC is:

The ratio of Vout to Vin is called the AC voltage gain (Av) and is expressed as follows:

This shows that the transistor in the circuit in the figure above provides amplification or voltage gain, dependent on the values of RC and re.

Common-Emitter Amplifiers:

In this example, the input is at the base, the output is at the collector and the emitter is common to both input and output (at a common ground)

  • An appropriate by-pass capacitor connected across RE increases the amplifier’s gain, as shown in the illustration below.
  • When the load resistance is connected to the output at the collector through a coupling capacitor, the voltage gain is reduced.
  • To achieve gain stability, a technique of partially by-passing RE is often used in order to reduce dependency on RE, though the gain is reduced.
  • The output voltage at the collector is 180° out of phase with the input voltage at the base. The phase inversion is sometimes indicated by a negative sign in front of the voltage gain.

Common-Collector Amplifiers:

In this example, the input is at the base, the output is at the emitter and the collector is common to both (at a common voltage source)

  • Usually referred to as an emitter-follower circuit.
  • The input is applied to the base through a coupling capacitor and the output is at the emitter.
  • There is no collector resistor.
  • The voltage gain is approximately 1.
  • Its main advantage is a high input resistance.
Common-Base Amplifiers:

In this example, the input is at the emitter, the output is at the collector and the base is common to both.

  • The least used of the three basic amplifier configurations.
  • Provides high-voltage gain with no current gain.
  • It has a low input resistance, so it is the most appropriate for certain high-frequency applications where sources tend to have very low-resistance output.

 

Biasing Example:

The Q point of the circuit shown in the following figure is defined by IC and VCE as follows:

Use of Transistors as Voltage Amplifiers !

The Q point is at IC = 39.6 mA and at VCE = 6.93 V. Since IC (cut-off) = 0, we need to know IC (sat) to determine how much variation in the collector current can occur and still maintain a linear operation of the transistor. All of the obtained values are shown in the DC load line of the circuit.

Use of Transistors as Voltage Amplifiers !

The following animation shows the effect of VCE being smaller, close, and bigger than the calculated VCE on the output signal. We will achieve an improper biasing of the circuit by modifying the voltage at the base VBB. As you can see in the animation, the simulated shown values are close to the ideally calculated ones. We start with an appropriate VBB´of 10 V, then we reduce it to 1 V and then we increase it to 16 V in order to see what happens with the output and measure it with the oscilloscope. The input is shown in yellow at channel 1 of the oscilloscope, while the output is shown in blue at channel 2 of the virtual oscilloscope.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

There are several ways to bias a transistor. In the previous example, a separate voltage source VBB was used to bias the base-emitter junction. Here, some other ways will be presented, each offering advantages and disadvantages:

Method

Advantage

Disadvantage

Base bias

Uses VCC as a single bias source

The variation of βdc causes both IC and VCE to change, thus changing the Q-point of the transistor and making the base-biased circuit β-dependent.

 

Emitter bias

The emitter current can be essentially independent of βdc and VEE >> VBE

 

It requires two separate DC voltage sources, one positive and one negative.

Voltage-divider bias

IEis essentially independent of βdc. The circuit provides good Q-point stability with a single-polarity supply. This is the most common type of biasing.

Requires more components and the stability is reached as long as RE > 10*[R1R2/(R1+R2)]

Collector-feedback bias

Provides good stability using negative feedback from collector to base and it is simple in the components required.

 

Summary of biasing methods and some of their advantages and disadvantages

Although here we basically concentrated on one type of transistor, either NPN or PNP, do not forget that the only difference is that the polarities of the supply voltages are reversed, but the principles for biasing are the same. The biasing configurations are shown in the following figure.

Use of Transistors as Voltage Amplifiers !

 

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