Analog Electronics In A Day Analog Electronic Design JFET Model Gate Drain R Source 1-32 When the JFET is biased in its linear region the gate is represented as an open circuit. The drain to source current is a voltage controlled current source,gm(eg). The output resistance is modeled by Ro. As long as the signal swings stay in the linear region, the gate-source voltage signal swing induces a drain -source current
1-32 Analog Electronics In A Day Analog Electronic Design 1-32 Source Gate eg RO Drain gmeg When the JFET is biased in its linear region the gate is represented as an open circuit. The drain to source current is a voltage controlled current source, gm(eg). The output resistance is modeled by RO. As long as the signal swings stay in the linear region, the gate-source voltage signal swing induces a drain-source current
Analog Electronics In A Day Analog Electronic Design Metal oxide semiconductor Field Effect transistor Comes in two flavors: N-channel and P-channel Highest input impedance Output circuit is low impedance Excellent power switches No second-breakdown failure mechanism Excellent bandwidth 1-33 The BJT and JFET have a diode in their input circuit which controls their mode of operation. The metal oxide semiconductor field effect transistor (MOSFET works on a similar principle, but the diode is buried within the MOSFET. The MOSFET input diode is controlled by an electric field in the gate region, thus the input impedance is extremely high because there are no diode effects to lower the impedance. The input impedance of MOSFETs is so high that there is no mechanism that can bleed off accumulated charge, thus they are packaged with lead shorting wires to protect them The MOSFET is a majority carrier device, and because majority carriers have no recombination delays, the MOSFET achieves extremely high bandwidths and switching times. The gate is electrically isolated from the source, and while this provides the MOSFET with its high input impedance it also forms a good capacitor. Driving the gate with a dc or low frequency signal is a snap because ZiN is so high, but driving the gate with a step signal is much harder because the gate capacitance much be charged at the signal rate. This situation leads to a paradox; the high input impedance MOSFET must be driven with a low impedance driver to obtain high switching speeds or low bandwidth MOSFETs do not have a secondary breakdown area, and their drain source resistance has a positive temperature coefficient, so they tend to be self protective. These features coupled with the very low on resistance and lack of a junction voltage drop when forward biased, make the MOSFET an extremely attractive power supply switching transistor
1-33 Analog Electronics In A Day Analog Electronic Design 1-33 • Comes in two flavors: N-channel and P-channel • Highest input impedance • Output circuit is low impedance • Excellent power switches • No second-breakdown failure mechanism • Excellent bandwidth The BJT and JFET have a diode in their input circuit which controls their mode of operation. The metal oxide semiconductor field effect transistor (MOSFET) works on a similar principle, but the diode is buried within the MOSFET. The MOSFET input diode is controlled by an electric field in the gate region, thus the input impedance is extremely high because there are no diode effects to lower the impedance. The input impedance of MOSFETs is so high that there is no mechanism that can bleed off accumulated charge, thus they are packaged with lead shorting wires to protect them. The MOSFET is a majority carrier device, and because majority carriers have no recombination delays, the MOSFET achieves extremely high bandwidths and switching times. The gate is electrically isolated from the source, and while this provides the MOSFET with its high input impedance it also forms a good capacitor. Driving the gate with a dc or low frequency signal is a snap because ZIN is so high, but driving the gate with a step signal is much harder because the gate capacitance much be charged at the signal rate. This situation leads to a paradox; the high input impedance MOSFET must be driven with a low impedance driver to obtain high switching speeds or low bandwidth. MOSFETs do not have a secondary breakdown area, and their drainsource resistance has a positive temperature coefficient, so they tend to be self protective. These features coupled with the very low on resistance and lack of a junction voltage drop when forward biased, make the MOSFET an extremely attractive power supply switching transistor
Analog Electronics In A Day Analog Electronic Design MOSFET Description Drain Drain Gate Gate Source Source N-Channel mosfet P-channel mosfet The MOSFET can be visualized as a bar of doped silicon that contains a diode junction in the middle of the bar. If the silicon bar is doped N, then the MOSFET is called a N-channel device. When the n-channel gate charged negative with respect to the source the internal gate diode is biased off, the bar is depleted of carriers, and the source to drain resistance is quite high (several MQ2). When the n-channel gate is charged positive with respect to the source the internal gate diode is biased on, the bar is flooded with carriers thus causing a low source to drain resistance (as low as mQ2). The converse is true for a P-channel MOSFET 1-34
1-34 Analog Electronics In A Day Analog Electronic Design 1-34 N-Channel MOSFET P-Channel MOSFET Drain Source Gate Drain Source Gate The MOSFET can be visualized as a bar of doped silicon that contains a diode junction in the middle of the bar. If the silicon bar is doped N, then the MOSFET is called a N-channel device. When the n-channel gate is charged negative with respect to the source the internal gate diode is biased off, the bar is depleted of carriers, and the source to drain resistance is quite high (several MΩ). When the n-channel gate is charged positive with respect to the source the internal gate diode is biased on, the bar is flooded with carriers thus causing a low source to drain resistance (as low as mΩ). The converse is true for a P-channel MOSFET
Analog Electronics In A Day Analog Electronic Design MOSFET Model Gate o Drain N Source 1-35 When the MOSFET is biased in it's linear region the gate is represented as an open circuit. The drain to source current is a voltage controlled current source, gm(eg). The output resistance is modeled by Ro. AS long as the signal swings stay in the linear region, the gate-source voltage signals induce a drain-source current The mosfet contains a diode connected across from the drain (cathode)to the source (anode). This diode is not forward biased during normal operation, consequently it does not conduct current during normal operation. When the MOSFET is connected to an inductive load the inductive kick causes the diode to turn on and conduct current. In some modes of operation this is a desired effect because it limits the inductive voltage rise. The diode is not fast turn-off, so it consumes quite a bit of power during turn-off. The turn-off power consumption is detrimental in some circuits, thus those circuits must put a diode with a smaller forward voltage drop schottky diode) in parallel with the body diode CIN can be as large as several hundred pF, and it must be charged by the gate signal. The gate is normally driven by a low impedance driver so that CIN can be charged quickly. If CIN is charged slowly, the switching time of the MOSFET is long, causing the MOSFET to stay in the linear region for a long time. When the MOSFET operates in the linear region it's voltage drop and current flow are high resulting in a high power dissipation
1-35 Analog Electronics In A Day Analog Electronic Design 1-35 Gate eg Source RO Drain CIN gmeg When the MOSFET is biased in it’s linear region the gate is represented as an open circuit. The drain to source current is a voltage controlled current source, gm(eg). The output resistance is modeled by RO. As long as the signal swings stay in the linear region, the gate-source voltage signals induce a drain-source current. The MOSFET contains a diode connected across from the drain (cathode) to the source (anode). This diode is not forward biased during normal operation, consequently it does not conduct current during normal operation. When the MOSFET is connected to an inductive load the inductive kick causes the diode to turn on and conduct current. In some modes of operation this is a desired effect because it limits the inductive voltage rise. The diode is not fast turn-off, so it consumes quite a bit of power during turn-off. The turn-off power consumption is detrimental in some circuits, thus those circuits must put a diode with a smaller forward voltage drop (Schottky diode) in parallel with the body diode. CIN can be as large as several hundred pF, and it must be charged by the gate signal. The gate is normally driven by a low impedance driver so that CIN can be charged quickly. If CIN is charged slowly, the switching time of the MOSFET is long, causing the MOSFET to stay in the linear region for a long time. When the MOSFET operates in the linear region it’s voltage drop and current flow are high resulting in a high power dissipation
Analog Electronics In A Day Analog Electronic Design Voltage Feedback Op Amp Excellent precision Made from bjt or fet Good bandwidth Very high initial ga Gain roll-off starts quickly High input impedance Low output impedance The voltage feedback operational amplifier (V op amp), or op amp as it is affectionately known, is a versatile amplifier which requires feedback to function. The op amp gain is so high that the output saturates on any differential input signal, so feedback is employed to lower the closed loop gain. The feedback makes the op amp circuit a precision circuit because the gain becomes dependent on the passive components which can be very accurate. Some op amp parameters, such as input offset voltage can still degrade the precision, but there are specially designed precision op amps that have very low input offset voltages(micro volts), and selected salient parameters chosen to yield a precision circuit Op amp bandwidth depends on the process used to make the op amp and Bjt op amps are highest bandwidth and current drain, with JFET op amps being next, and MoSFET op amps have the lowest bandwidth and current drain. Voltage feedback op amps are discussed in this section, and their bandwidth starts rolling off at low frequencies (about five decades before the advertised gain-bandwidth point The input impedance of the VF op amps is very high, and their output impedance is relatively low, thus they ideal for configuring many different circuits. Some the possible circuits op amp can make are inverting amplifiers, non-inverting amplifiers, differential amplifiers summing amplifiers, integrating amplifiers, and a host of others
1-36 Analog Electronics In A Day Analog Electronic Design 1-36 • Excellent precision • Made from BJT or FET • Good bandwidth • Very high initial gain • Gain roll-off starts quickly • High input impedance • Low output impedance The voltage feedback operational amplifier (VF op amp), or op amp as it is affectionately known, is a versatile amplifier which requires feedback to function. The op amp gain is so high that the output saturates on any differential input signal, so feedback is employed to lower the closed loop gain. The feedback makes the op amp circuit a precision circuit because the gain becomes dependent on the passive components which can be very accurate. Some op amp parameters, such as input offset voltage can still degrade the precision, but there are specially designed precision op amps that have very low input offset voltages (micro volts), and selected salient parameters chosen to yield a precision circuit. Op amp bandwidth depends on the process used to make the op amp, and BJT op amps are highest bandwidth and current drain, with JFET op amps being next, and MOSFET op amps have the lowest bandwidth and current drain. Voltage feedback op amps are discussed in this section, and their bandwidth starts rolling off at low frequencies (about five decades before the advertised gain-bandwidth point. The input impedance of the VF op amps is very high, and their output impedance is relatively low, thus they ideal for configuring many different circuits. Some the possible circuits op amp can make are inverting amplifiers, non-inverting amplifiers, differential amplifiers, summing amplifiers, integrating amplifiers, and a host of others