N channel JFET: ◦ Major structure is n-type material (channel) between embedded p-type material to form 2 p-n junction. ◦ In the normal operation of an n-channel device, the Drain (D) is positive with respect to the Source (S). Current flows into the Drain (D), through the channel, and out of the Source (S) ◦ Because the resistance of the channel depends on the gate-to-source voltage (V ), the drain current (I ) is controlled by that voltage GS D
P-channel JFET P channel JFET: ◦Major structure is p-type material (channel) between embedded n-type material to form 2 p-n junction. ◦Current flow : from Source (S) to Drain (D) ◦Holes injected to Source (S) through p-type channel and flowed to Drain (D)
JFET Characteristic for V | GS = 0 V and 0<VDS<|Vp To start, suppose V =0 GS Then, when V is increased, I increases. Therefore, I is proportional to DS D D V for smal values of V DS DS For larger value of V , as V increases, the depletion layer become DS DS wider, causing the resistance of channel increases. After the pinch-off voltage (V ) is reached, the I becomes nearly constant p D (cal ed as I maximum, I -Drain to Source current with Gate Shorted) D DSS
JFET for V = 0 V and 0<V <|V | GS DS p
Pinch-off (V = 0 V, V = V ). GS DS P
ID versus VDS for VGS = 0 V and 0<VDS<|Vp|
JFET for (Application of a negative voltage to the gate of a JFET)
JFET Characteristic Curve For negative values of V , the gate-to-channel junction is reverse biased even with GS V =0 DS Thus, the initial channel resistance of channel is higher. The resistance value is under the control of VGS If V = pinch-off voltage(V ) GS P The device is in cutoff (V =V = V ) GS GS(off) P The region where I constant – The saturation/pinch-off region D The region where I depends on V is called the linear/ohmic region D DS
OPERATING IF JFET:SATURATING REGION
OPERATING OF JFET:OHMIC REGION
p-Channel JFET characteristics with IDSS = 6 mA and VP = +6 V.
Characteristics for n-channel JFET & p-channel JFET The behavior of a JFET can be described in terms of a set of Characteristic Curves shown here. In the region shown with a green background the drain-source voltage is small and the channel behaves like a fairly ordinary conductor. In this region the current varies roughly in proportion to the drain-source voltage as if the JFET obeys Ohm's law. However, as we increase the drain-source voltage and move into the region with a light background we increase the drain- channel voltage so much that we start to ‘squeeze down’ the channel.
IV CURVER OF JFET
BASICE JFET AMPLIFIER common source common drain common gate -Each circuit configuration describes a two port network having an input and an output. The transfer function of each is also determined by the input and output voltages or currents of the circuit.
APPLICATION OF JFET AS AMPILFIER Low-Noise Amplifier Differential Amplifier Constant Current Source Analog Switch or Gate Voltage Controlled Resistor
Low-Noise Amplifier A minor change to the circuit of Figure 3 describes a basicsingle stage low-noise JFET amplifier. Figure 4 shows that this change only incorporates a resistor from the gate to Vss. This resistor supplies a path for the gate leakage current in an AC coupled circuit. Its value is chosen by the required input impedance of the amplifier and its desired low-noise characteristics. The noise components of this amplifier are the thermal noise of the drain and gate resistors plus the noise components of the JFET. The noise contribution of the JFET is from the shot noise of the gate leakage current, the thermal noise of the channel resistance, and the frequency noise of the channel. These noise characteristics are generally lower than those found in bipolar transistors if the JFET is properly selected for the application. The voltage gain of the circuit is again defined by Equation ).
Differential Amplifier Another application of the JFET is the differential amplifier. This configuration is shown in Figure 5. The differential amplifier requires that the two transistors be closely matched electrically and physically located near each other for thermal stability. Either input and either output can be used or both inputs and only one output and conversely only one input and both outputs can be used. For the configuration shown the source resistor is chosen to determine the gate to source bias voltage, remembering that the current wil be twice that of each of the JFET drain currents. The value of the drain resistors is chosen to provide a suitable dynamic range at the output/ The gain of this circuitis defined by: (5) AV = 2x (gm x Rl) / (1 + gm x RS ) where all the terms in the equation have previously been defined.
Constant Current Source A constant current source using a JFET is shown in Figure 6. This circuit configuration has many useful applications ranging from charging circuits for integrators or timers to replacing the source resistor in the differential amplifier shown in Figure 5. The current provided by the constant current source of Figure 6 is defined as (6) ID = IDSS [ 1 - ( VGS / Vp) ] 2 where ID = the drain current or magnitude of current sourced IDSS = the drain saturation current of the JFET VGS = ID x RS Vp = the JFET pinch-off voltage 2 = the squared value of the term in brackets. 01/99 H-7 1000
Analog Switch or Gate Figures 7, 8, and 9 show three different applications for the JFET to be used as an analog switch or gate. Figures 7 and 8 both demonstrate methods for realizing programmable gain amplifiers, while Figure 9 shows an analog multiplexer circuit using JFETs and a common op-amp integrated circuit. It can be seen from Figure 7 that the gain of the stage can be changed by switching in any combination of feedback resistors R1 through Rn. The JFET in series with the input resistor should be of the same type as those in the feedback paths and is used for thermal stability of the circuit gain. The transfer function of the circuit of Figure 7 is approximated by:
The circuit of Figure 8 shows another method to realize a programmable gain amplifier using a common op-amp, four resistors, and only two JFETs. The gain of this circuit can also be changed by switching in the desired resistors by turning off the appropriate JFET thus switching in the paral el resistor. The transfer function of this circuit is approximated by: (8) Vo / Vi = (R3 + R4) / (R1 + R2)
It should be noted that only those resistors which are switched into the circuit are to be included in it should be noted that only those resistors which the transfer function equation. Figure 9 shows a circuit in which the JFETs are acting as analog switches to multiplex several input signal sources to a single output source. The transfer function of this circuit is then approximated by: (9) Vo / Vi = Rf / Rn where Rf = the feedback resistor Rn = any one of the input resistors Further examination of this circuit shows that it can also be used as a programmable summing amplifier by switching in any combination of input signals. The transfer function is then approximated by: (10) Vo / Vi = (Rf / R1) + (Rf / R2) + .... + (Rf /Rn) Again in this application only those resistors which are switched into the circuit are to be included in the transfer function equation
Voltage Controlled Resistor Another common application for the JFET is as a voltage controlled resistor. The JFET action in normal operation simply changes the cross sectional dimensions of the channel. When the JFET is biased in the resistive or linear region as shown in Figure 10, a change in gate voltage and the corresponding change in channel dimensions simply changes the drain to source resistance of the device.
TYPES OF AMPLIFIER •Class A Amplifier •Class B Amplifier •Class AB Amplifier •Class C Amplifier
Class A Amplifier The output device (transistor) conducts electricity for the entire cycle of input signal. In other words, they reproduce the entire waveform in its entirety. These amps run hot, as the transistors in the power amp are on and running at full power al the time. There is no condition where the transistor(s) is/are turned off. That doesn't mean that the amplifier is never or can never be turned off; it means the transistors doing the work inside the amplifier have a constant flow of electricity through them. This constant signal is called "bias". Class A is the most inefficient of al power amplifier designs, averaging only around 20.
Class B Amplifier The input signal has to be a lot larger in order to drive the transistor appropriately. This is almost the opposite of Class A operation There have to be at least two output devices with this type of amp. This output stage employs two output devices so that each side amplifies each half of the waveform. [li Either both output devices are never allowed to be on at the same time, or the bias (remember, that trickle of electricity?) for each device is set so that current flow in one output device is zero when not presented with an input signal. Each output device is on for exactly one half of a complete signal cycle
Class AB Amplifier In fact, many Class AB amps operate in Class A at lower output levels, again giving the best of both worlds The output bias is set so that current flows in a specific output device for more than a half the signal cycle but less than the entire cycle. There is enough current flowing through each device to keep it operating so they respond instantly to input voltage demands. In the push-pull output stage, there is some overlap as each output device assists the other during the short transition, or crossover period from the positive to the negative half of the signal.
Class C Amplifier The Class C Amplifier design has the greatest efficiency but the poorest linearity of the classes of amplifiers mentioned here the class C amplifier is heavily biased so that the output current is zero for more than one half of an input sinusoidal signal cycle with the transistor idling at its cut-off point. In other words, the conduction angle for the transistor is significantly less than 180 degrees, and is general y around the 90 degrees area. this form of transistor biasing gives a much improved efficiency of around 80% to the amplifier, it introduces a very heavy distortion of the output signal. Therefore, class C amplifiers are not suitable for use as audio amplifiers. class C amplifiers are commonly used in high frequency sine wave oscil ators and certain types of radio frequency amplifiers, where the pulses of current produced at the amplifiers output can be converted to complete sine waves of a particular frequency by the use of LC resonant circuits in its col ector circuit.
Amplifier Classes and Efficiency As well as audio amplifiers there are a number of high efficiency Amplifier Classes relating to switching amplifier designs that use different switching techniques to reduce power loss and increase efficiency. Some amplifier class designs listed below use RLC resonators or multiple power-supply voltages to reduce power loss, or are digital DSP (digital signal processing) type amplifiers which use pulse width modulation (PWM) switching techniques
DC LOAD LINE Consisder a CE amplifier along with the output characteristics as shown in figure 3.18 above. A straight line drawn on the output characteristic of a transistor which gives the various zero signal values (ie. When no signal applied) of VCE and IC is cal ed DC load line Construction of DC load line Applying KVL to the col ector circuit we get, VCC–ICRC –VCE =0-------------------1 VCE = VCC –ICRC ----------------------2 The above equation is the first degree equation and can be represented by a straight line. This straight line is DC load line. To draw the load line we require two end points which can be found as fol ows. If IC =0, equn 2 becomes VCE = VCC if VCE = 0, equn 2 becomes VCC = ICRC ie. IC = VCC /RC