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Analog Electronics Lecture 1_Independent and Controlled Sources.

Module by: Bijay_Kumar Sharma. E-mail the author

Summary: A battery or power socket in our household is an independent source of electric energy. The output of this source is independent of any other electrical parameter in the circuit in which the independent source is being used. Active Devices such as Vacuum Triode, Tetrode, Pentode, Field Effect Transistor(FET), Bipolar Junction Transistor(BJT) and Operational Amplifier(Op.Amp.) are controlled sources. Vacuum Triode, Tetrode, Pentode, Field Effect Transistor(FET) and Op.Amp. are Voltage Controlled Sources whereas BJT is Current Controlled Source. In Vacuum Tubes input voltage at the grid, in Common Source FET input voltage at the gate and in Op. Amp. differential voltage at the double ended input decide the output voltage/current. Hence Vacuum Tubes, FET and Op Amp are Voltage Controlled Sources. In case of BJT, input current flowing in the base of Common Emitter BJT decide the output electrical parameter hence BJT is Current Controlled Source. Now any Power Source can be represented as Voltage Source by drawing the Thevenin Equivalent Circuit of the given Power Source and as Current Source by drawing the Norton Equivalent Source. A source with large internal impedance or source impedance is drawn as a Current Source. When internal impedance is infinite then we have an Ideal Current Source. An Ideal Current Source is a Constant Current Source and it supplies constant terminal current irrespective of the terminal voltage. Similarly an Ideal Voltage Source has zero internal impedance. It gives Constant Terminal Voltage irrespective of load current. In real life we cannot have Ideal Voltage Source or Ideal Current Source. Any Voltage Source will have a finite and small internal impedance. Because of this as the terminal current increases, the terminal voltage starts drooping. This is called Regulation. In unregulated Power Supply there is considerable power droop with high load current as in our household power supplies. As a Tulu Pump is turned on, the house hold supply voltage drops and incandescent lamps are dimly lit. This is because of finite internal impedance of household supply. Since there are two power source representations such as Voltage Source and Current Source hence Controlled Sources are of four varieties: Voltage Controlled Voltage Source(VCCS)- Triode & Op.Amp; Voltage Controlled Current Source(VCCS)- Pentode and FET; Current Controlled Voltage Source(CCVS); Current Controlled Current Source(CCCS)- BJT; The incremental or small signal models of these four Controlled Sources are described.

Analog Electronics Lecture 1_Independent and Controlled Sources.

Key words: Ideal Voltage Source, Thevenin Equivalent Circuit, incremental model.

Abstract:

A battery or power socket in our household is an independent source of electric energy. The output of this source is independent of any other electrical parameter in the circuit in which the independent source is being used. Active Devices such as Vacuum Triode, Tetrode, Pentode, Field Effect Transistor(FET), Bipolar Junction Transistor(BJT) and Operational Amplifier(Op.Amp.) are controlled sources. Vacuum Triode, Tetrode, Pentode, Field Effect Transistor(FET) and Op.Amp. are Voltage Controlled Sources whereas BJT is Current Controlled Source. In Vacuum Tubes input voltage at the grid, in Common Source FET input voltage at the gate and in Op. Amp. differential voltage at the double ended input decide the output voltage/current. Hence Vacuum Tubes, FET and Op Amp are Voltage Controlled Sources. In case of BJT, input current flowing in the base of Common Emitter BJT decide the output electrical parameter hence BJT is Current Controlled Source. Now any Power Source can be represented as Voltage Source by drawing the Thevenin Equivalent Circuit of the given Power Source and as Current Source by drawing the Norton Equivalent Source. A source with large internal impedance or source impedance is drawn as a Current Source. When internal impedance is infinite then we have an Ideal Current Source. An Ideal Current Source is a Constant Current Source and it supplies constant terminal current irrespective of the terminal voltage. Similarly an Ideal Voltage Source has zero internal impedance. It gives Constant Terminal Voltage irrespective of load current. In real life we cannot have Ideal Voltage Source or Ideal Current Source. Any Voltage Source will have a finite and small internal impedance. Because of this as the terminal current increases, the terminal voltage starts drooping. This is called Regulation. In unregulated Power Supply there is considerable power droop with high load current as in our household power supplies. As a Tulu Pump is turned on, the house hold supply voltage drops and incandescent lamps are dimly lit. This is because of finite internal impedance of household supply. Since there are two power source representations such as Voltage Source and Current Source hence Controlled Sources are of four varieties:

Voltage Controlled Voltage Source(VCCS)- Triode & Op.Amp;

Voltage Controlled Current Source(VCCS)- Pentode and FET;

Current Controlled Voltage Source(CCVS);

Current Controlled Current Source(CCCS)- BJT;

The incremental or small signal models of these four Controlled Sources are described.

LECTURE NO.1

Q.1)Define Independent and Controlled/Dependent sources.

Ans:-Those sources which do not depend upon the values of the current or the voltages in any other part of that circuit are called independent sources.

Those sources which depend upon the values of either voltage or current at any part of that circuit are called dependent (or) controlled sources.

Q.2)Define independent voltage source and independent current source.

Ans:-An independent voltage source is a Thevenin Equivalent of power source as shown below:

Figure 1
Figure 1 (Picture 2.png)

An independent current source is Norton Equivalent of power source as shown below:

Figure 2
Figure 2 (Picture 3.png)

Q.3)When do we choose Thevenin Equivalent and when do we choose Norton Equivalent?

Ans:- When the internal impedence or the source impedance of the power source is low ,we choose Thevenin’s Equivalent and when the internal impedence of the power source is high ,we choose Norton’s Equivalent.

Q.4)Define Ideal Voltage and Ideal current sources.

Ans:- An ideal voltage source is that source which :

  1. a) Has Zero internal impedence.
  2. b) 0% Regulation i.e. it gives a constant terminal voltage irrespective of the load current.

An ideal current source is that source which:

  1. a) Has Infinite internal impedence.
  2. b) Gives constant terminal current irrespective of the load voltage.

Q.5) what are the types of controlled sources?

Ans:-The types of CONTROLLED sources are:-

  1. a) Voltage Controlled Voltage Source
  2. b) Voltage Controlled Current Source
  3. c) Current Controlled Voltage Source
  4. d) Current Controlled Current Source

The controlled sources shown in the figure are ideal. In practice we do not have ideal controlled sources hence Rout of the controlled sources must be shown. Also at the input Rin will have to be shown.

In each of the controlled sources Rin and Rout should be shown as indicated in the Table below.

Table 1. Set of Rin and Rout and their placements in the four controlled sources.

Table 1
Type of controlled source Input impedance (Rin) Output Impedance(Rout)
VCVS High impedance at I/P node pair Low impedance in series with controlled voltage source
VCCS High impedance at I/P node pair High impedance in parallel with controlled current source
CCVS Low impedance at I/P node pair Low impedance in series with controlled voltage source
CCCS Low impedance at I/P node pair High impedance in parallel with controlled current source

Table 2
Type Of Controlled Source Rin Rout Controlled Source Controlling Parameter Type Of Ideal Amplifier Parameter Example Of Such A Device
Voltage Controlled Voltage Source ∞ 0 Voltage Input Voltage Voltage Amplifier Av Triode,Op-Amp
Voltage Controlled Current Source ∞ ∞ Current Input Voltage Transconductance Amplifier Gm Pentode,Field Effect Transistor(FET)
Current Controlled Voltage Source 0 0 Voltage Input Current Transresistance Amplifier Rm __
Current Controlled Current Source 0 ∞ Current Input Current Current Amplifier AI Bipolar Junction Transistor(BJT)

The incremental model of the four controlled sources are given below.

Under no signal condition when only DC currents and DC voltages are correlated through a Model.

Under signal condition, there are instantaneous voltages and currents flowing in the circuit.

Instantaneous Parameter – DC parameter = Incremental parameter.

The Model which correlates the incremental parameters under signal condition is known as Incremental Model. In circuit analysis of electronic circuits we always use incremental models . Hence generally we will give incremental parameters unless otherwise stated

Figure 3
Figure 3 (Picture 4.png)
Figure 4
Figure 4 (Picture 8.png)
Figure 5
Figure 5 (Picture 6.png)
Figure 6
Figure 6 (Picture 7.png)

Figure 7
Figure 7 (Picture 9.png)

Figure 8
Figure 8 (Picture 10.png)

As seen from the I-V characteristics, Ideal Voltage Source is a vertical line whereas Ideal Current Source is a horizontal line.

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