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What is a signal/ function generator?
A function generator is a piece of electronic test equipment used to
generate electric waveform.The resultant waveforms can be injected
into a device under test and analyzed as they progress through the
device, confirming the proper operation of the device or pinpointing
a fault in it. Thus it forms a basic component in any electronics lab.
The function generator provides sinusoidal, square, ramp, sawtoothed waveform of required frequency as its output.
CIRCUIT DIAGRAM add here
Brief explanation of working
The frequency can be changed by using FREQUENCY CONTROL
on the front panel or by externally applied control voltage. Actually
the frequency control voltage regulates the two current sources as
shown in the circuit diagram.
The upper current source supplies a constant current to the
integrator whose output increases linearly with time. The output
voltage is given by
text/image add eqn here
The voltage comparator multivibrator changes state at a
predetermined level on the positive slope of the integrator's output
voltage. This change of state cuts off the upper current supply to the
integrator and switches on the current supply.
The lower current supply provides a reverse current to the integrator
so that its output voltage decreases linearly with time. When the
output voltage reaches a predetermined level on the negative slope of
the output waveform the voltage comparator changes state and cuts
off the lower current source again.
The voltage at the output of the integrator has a triangular
waveform whose frequency is controlled by current supplied by the
constant current sources. The comparator provides a square wave
output of the same frequency. The triangular waveform is converted
into a sine wave by shaping circuit formed by diodes & resistance
network.
The two output amplifiers are used to provide two output
waveforms simultaneously.
So the whole circuit can be split up in the following parts:
1. frequency control network
2. Current sources
3. Opamp as integrator
4. Voltage comparator multivibrator
5. Resistance diode shaping network
6. Opamp as buffer
Now we will be explaining these parts one by one with details
An ideal current source I, driving a resistor R, and creating a voltage V
A current source is an electrical or electronic device that delivers or absorbs
electric current. Current sources can be theoretical or practical. This page covers
both theoretical and practical forms of current source.
Theoretical current sources
An ideal current source is a conceptual source used in network theory and
analysis that delivers or absorbs electrical energy such that the electric current is
independent of the voltage across its terminals. The voltage across an ideal
current source is completely determined by the circuit connected to the source.
Ideal current sources are not found in nature, although many electronic devices,
such as transistors are modelled as non ideal dependent current sources. Most
current sources in electrical network theory are treated as non-ideal. That is, they
have finite output impedance.
Practical current sources
Sources using active devices
Active current sources have many important applications in electronic circuits.
Current sources are often used in place of resistors in analog integrated circuits
to generate a current without causing attenuation at a point in the signal path to
which the current source is attached. The collector of a bipolar transistors, the
drain of a field effect transistors, or the plate of a vacuum tubes naturally behave
as current sources when properly connected to an external source of energy
(such as a power supply) because the output impedance of these devices is
naturally high when used in the current source configuration.
JFET and N-FET current source
A JFET can be made to act as a current source by tying its gate to its source. The
current then flowing is the IDSS of the FET. These can be purchased with this
connection already made and in this case the devices are called current regulator
diodes. An enhancement mode N channel MOSFET can be used in the circuits
listed below.
Simple transistor current source
Typical constant current source (CCS)
DZ1 is a zener diode which, when reverse biased (as shown in the circuit) has a
constant voltage drop across it irrespective of the current flowing through it.
Thus, as long as the zener current (IZ) is above a certain level (called holding
current), the voltage across the zener diode (VZ) will be constant. Resistor R1
supplies the zener current and the base current (IB) of NPN transistors (Q1). The
constant zener voltage is applied across the base of Q1 and emitter resistor R2.
The operation of the circuit is as follows:
Voltage across R2 (VR2) is given by VZ - VBE, where VBE is the base-emitter drop of
Q1. The emitter current of Q1 which is also the current through R2 is given by
Since VZ is constant and VBE is also constant for a given temperature, it follows
that VR2 is constant and hence IE is also constant. Due to transistor action, IE is
very nearly equal to the collector current IC of the transistor (which in turn, is the
current through the load). Thus, the load current is constant and the circuit
operates as a constant current source. As long as the temperature remains
constant (or doesn't vary much), the load current will be independent of the
supply voltage, R1 and the transistor's gain. R2 allows the load current to be set
at any desirable value and is calculated by
Or
, since VBE is typically 0.65 V for a silicon device.
(IR2 is also the emitter current and is assumed to be the same as the collector or
required load current, provided hFE is sufficiently large). Resistance R1 at resistor
R1 is calculated as
Where, K = 1.2 to 2 (so that R1 is low enough to ensure adequate IB),
And hFE(min) is the lowest acceptable current gain for the particular transistor type
being used.
Simple transistor current source with diode compensation
Typical constant current source (CCS) with diode compensation
Temperature changes will cause the above circuit to change the output current
since VBE is sensitive to temperature. This can be compensated for by including a
standard diode D (of the same semiconductor material as the transistor) in series
with the zener diode as shown in the image on the left. The diode drop (VD)
tracks the VBE changes due to temperature and thus suppresses temperature
dependence of the CCS.
Resistance R2 is now calculated as
Since VD = VBE = 0.65 V,
Therefore,
(In practice VD is never exactly equal to VBE and hence it only suppresses the
change in VBE rather than nulling it out.)
and R1 is calculated as
(The compensating diode's forward voltage drop VD appears in the equation and
is typically 0.65 V for silicon devices.)
This method is most effective for zener diodes rated at 5.6 V or more. For
breakdown diodes of less than 5.6 V, the compensating diode is usually not
required because the breakdown mechanism is not as temperature dependent as
it is in breakdown diode above this voltage.
Simple transistor current source with LED
Typical constant current source (CCS) using LED instead of zener
Another method is to replace the zener diode with a light emitting diode LED1
as shown in the image on the left. The LED voltage drop (VD) is now used to
derive the constant voltage and also has the additional advantage of tracking
(compensating) VBE changes due to temperature. R2 is calculated as
And R1 as
, where ID is the LED current.
NOW, in our circuit diagram the there are two current sources, namely, upper
current source and lower current source.
The purpose of the upper current source is to provide current in forward direction
when switched ON by the voltage comparator multivibrator. Similarly the
purpose of the lower current source is to provide current in reverse direction
when switched ON by the voltage comparator multivibrator.
OPERATIONAL AMPLIFIER as an integrator has been used as to generate a
triangular output in our circuit.
An integrator is a device to perform the mathematical operation known as
integration.
Equation and waveform
diagram to be added
here
OPERATIONAL AMPLIFIER AS BUFFER OR VOLTAGE FOLLOWER
Used as a buffer amplifier, to eliminate loading effects or to interface impedances
(connecting a device with a high source impedance to a device with a low input
impedance).
Waveform to be added
In our circuit we are using this circuit at the final output.