Common Resource JFET Amplifier
The amplifier circuit consists of an N-channel JFET, but since the circuit diagram will be the same as a change in FET connected in a common welding configuration, the device can also be an equivalent N-channel exhaustion mode MOSFET. The JFET gate voltage Vg is polarized through the potential dividing network established by resistors R1 and R2 and is polarized to operate within the saturation zone, which is equivalent to the active zone of the bipolar connection transistor. Unlike the bipolar transistor circuit, the FET connection receives almost no input gate current, which allows the door to be treated as an open circuit. Then you do not need the characteristic curves of input. We can compare JFET with the bipolar connection transistor (BJT) in the table below.
JFET and BJT Comparison
|Gate, ( G )||Base(Base), ( B )|
|Drain(Channel), ( D )||Collector(Collector), ( C )|
|Source, ( S )||Emitter, ( E )|
|Door Voltage, ( VG )||Base Voltage, ( VB )|
|Channel Voltage, ( VDD )||Collector Voltage, ( VCC )|
|Channel Current, ( ID )||Collector Stream, ( IC )|
Since the N-Channel JFET is a depletion mode device and is normally "ON", a negative door voltage by source is required to modulate or control the discharge current. This negative voltage can be achieved by polarizing or self-polarizing arrangement from a separate power supply voltage, as long as a constant current flows through the JFET even when there is no input signal and the Vg maintains the reverse polarity of the door-weld pn. In our simple example, polarization is provided by a potential dividing network that allows the input signal to produce a voltage drop at the door with a sinusoidal signal, as well as voltage rise at the door. Any pair of suitable resistance values of the right proportions will produce the right pre-voltage voltage, so dc gate pre-voltage voltage Vg is given as follows: Note that this equation only determines the ratio of R1 and R2 resistors, but we need to make these resistance values as high as possible to take advantage of JFET's very high input impedance and reduce the power loss in the circuit. Values from 1MΩ to 10MΩ are common. The input signal (Vin) of the common welding JFET amplifier is applied between the door terminal and the zero volt rail (0v). With a constant Vg gate voltage value, JFET operates within the "Ohmik zone", which acts as a linear resistant device. The discharge circuit includes rd load resistance. The output voltage, Vout, has been improved throughout this load resistance. The efficiency of the common welding JFET amplifier can be improved by adding a resistance, Rs, which is included in the welding cable with the same drainage current flowing from this resistance. Resistance, Rs is also used to set JFET amplifiers to "Q-point". When the JFET is fully positioned in the "ON" position, a voltage drop equal to Rs*Id develops during this resistance, and the potential of the welding terminal rises above 0v or soil level. This voltage drop along the Rs due to drainage current provides the necessary reverse polarization condition throughout the door resistance, R2 effectively produces negative feedback. Therefore, in order to keep the door-welding connection inverted, the welding voltage must be higher than the door voltage of vs, Vg. This source voltage is therefore given as follows: Then the Drainage current, Id is also equal to the Source current, enters the "No Current" Door terminal as Is, and this can be given as follows: This potentially divisive polarization circuit increases the stability of the common source JFET amplifier circuit while feeding from a single DC source compared to the fixed voltage polarization circuit. Both the resistance, Rs and weld bypass capacitor, Cs, basically, serve the same function as the emitter resistance and capacitor in the common emitter bipolar transistor amplifier circuit, that is, to ensure good stability and to prevent a decrease in voltage gain loss. However, the price paid for a stabilized silent door voltage is that more of the feed voltage is reduced to Rs. The value of the weld bypass capacitor in farads is usually quite high above 100uF and will be polarized. This gives the capacitor a much smaller impedance value of less than 10% of the transconductance, gm (transfer coefficient representing gain) value of the device. At high frequencies, the bypass capacitor essentially acts as a short circuit, and the welding will effectively be connected directly to the soil. The basic circuitry and properties of the Co-Welded JFET Amplifier are very similar to that of the common emitter amplifier. A DC load line is created by combining two points related to drainage current, Id and feed voltage, when Vdd is Id = 0: ( Vdd = Vds ) and Vds = 0: ( Id = Vdd/RL ). Therefore, the load line is the intersection of curves at point Q, as follows.
Common Source JFET Amplifier Characteristic Curve
As with the common emitter bipolar circuit, the DC load line for the common source JFET amplifier produces a straight line equation given as -1/(Rd + Rs) that is equal to the vertical Id axis at point A. Vdd/(Rd + Rs). The other end of the load line cuts the horizontal axis at point B, whose supply voltage is equal to Vdd. The actual position of the Q point in the DC load line is usually positioned at the midcentrist point of the load line (for class A operation) and is determined by the average value of the negatively prone Vg because the JFET is one. exhaustion mode device. Like the bipolar common emitter amplifier, the output of the Common Source JFET Amplifier is out of phase 180o with the input signal. One of the main drawbacks of using depletion mode JFET is that they need to be negatively biased. If this bias fails for any reason, the gate weld voltage may rise and become positive, causing an increase in the discharge current, causing the discharge voltage (Vd) to fail. In addition, the high channel resistance of the FET connection, rds(on), coupled with the high silent constant state drainage current, allows these devices to heat up, so additional coolant is required. However, most problems with the use of FETEs can be greatly reduced by using development mode MOSFET devices instead. MOSFETs or Metal Oxide Semiconductor FETs have much higher input impedance and low channel resistance compared to equivalent JFET. In addition, the polarization arrangements for MOSFETs are different, and unless you incline them positively for N-channel devices and negative for P-channel devices, no drainage current flows, then we actually have a fault-safe transistor.
Current and Power Gain of JFET Amplifier
We have previously said that the input current of a common source JFET amplifier, Ig, is too small due to the extremely high door impedance Rg. A common welding JFET amplifier therefore has a very good ratio between input and output impedances, and for any amount of output current, the IOUT of the JFET amplifier will have very high current gain Ai. Due to this common source, JFET amplifiers are extremely valuable as impedance matching circuits or are used as voltage amplifiers. Similarly, because: Power = Voltage x Current, (P = V*I) and output voltages are usually several millivolts or even volts, the power gain is also very high in ap. In the next lesson, we will look at how the transistor amplifier can cause disruption in the wrong front polarization voltage, in the form of amplitude deterioration due to clipping, as well as due to phase and frequency distortion.