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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.