Electronic Troubleshooting. Chapter 4 Field Effect Transistors

Transcription

1 Electronic Troubleshooting Chapter 4 Field Effect Transistors

2 Characteristics Common type of transistor, just like the bipolor ones covered in the previous section Use primarily where extremely high input impedance is required Most of today's transistors are "MOS-FETs", or Metal Oxide Semiconductor Field Effect Transistors. Ref: Types Covered JFET MOSFET VMOSFET

3 JFET Field Effect Transistors Theory of Doped Silicon Voltage across a lightly Doped silicon Small current flows Electrons enter the silicon through the Source Electrons exit through the Drain Bar of silicon acts as a resistor. Resistance dependent upon» Amount of impurities (doping in the silicon) in the silicon bar» Length and cross sectional area if the silicon bar

4 JFET Field Effect Transistors Theory of Doped Silicon w/gate PN Junction on the side of the bar Called a Gate The area around the gat has few electrons and is called the depletion region» Acts as a good insulator Reverse Biasing the PN Junction Increases the size of the depletion region Increases the resistance of the silicon bar The more reverse biasing voltage the greater the resistance Thus the current through the silicon bar can be varied by varying the biasing voltage

5 JFET Comparison parameter - Transconductance (g m ) Ratio of change in drain current to a change in gate-to-source voltage Units: mho Typically in µmho Example: A 2v change in V GS causes a 5mA change in I D g m I V D GS g m Find: g m I V D GS 5 ma V 2500 mho Circuit Symbols

6 JFET Circuit Symbols Arrows indicate direction of conventional current flow, thus the polarity required to reverse bias the junction Characteristic curve for a typical N-channel JFET Above 4 V V DS and V GS =0V I D is at its max value I D is controlled by changes in V GS Sample Problem Use chart and determine g m when changes from -2 to -3V g m I V D GS ( 4 2 ) ma ( 3 2 ) V 2000 mho

7 JFET Biasing A separate supply could be used to bias the JFET but self-biasing by using a Source resistor is more economical Sizing the Source resistor (S R ) Sample Problem: Given: Desired V GS = -3V and I D =2mA FIND: S R In Class exercises Page 83-4: 4-2, 4-5, 4-7, 4-9 V RS I GS D R S VGS 3V 1500 I 2mA D

8 JFET as an AC Amplifier Example Circuit With the circuit shown and the desire to prevent signal clipping Assume I D = 2mA THEN V D = V DD - I D R D = 10V The input signal V S is coupled through C C g m I V D GS i v d gs g v i v i R then and o d D m gs d then v o g m v gs R D and A v v v o gs g m R D

9 JFET as an AC Amplifier Example Circuit With the previous circuit shown and gm = 3000µmho A v v v o gs Add a load Resistor and coupling Cap The Gain changes to A V = g m r L R L and R D are in parallel for AC signals Input resistance Since the Gate-to-Source resistance is so high the Amp resistance matches the resistor just before the gate g m R D 6 5k 15

10 JFET as an AC Amplifier In-class Exercises Page 84. Problems: 4-10 and 4-12 Metal Oxide Semiconductor Field Effect Transistor (MOSFET) Current through the device is controlled by the Gate voltage Construction is similar to IC construction in that the various layers are individually deposited

11 MOSFET Types Depletion Mode Enhanced Mode Most Common Covered in this course Layers (continued) N+ layers are heavily doped N- areas are lightly doped Operation W/ 0volts on the gate the isn t enough electrons in the space between the source and drain for any significant current to flow

12 MOSFET Operation W/ + voltage on the gate electrons from the substrata are drawn into the channel between the Drain and Source This enhancement makes the channel conductive Current flows between the Source and Drain Enhanced Mode Device No current flows through the gate The more positive the Gate voltage the higher the current

13 MOSFET Symbols

14 MOSFET As Small Signal Amplifier Practically no drain current flows w/ 0V on gate V O depends upon: How high the gate voltage goes Value of R D How low of resistance that the MOSFET appears to have» R D and the MOSFET act as a voltage divider

15 MOSFET As Small Signal Amplifier Practically no drain current flows w/ 0V on gate Typical With a reasonable digital input of 5V» V O would = almost of zero With a reasonable input of 0V» V O would = almost 15V The range is much better than for a bipolar transistor

16 Power MOSFET Field Effect Transistors Typical non-power MOSFET s develop only a narrow channel between Source and Drain Typical only useful in low power applications due to the resistance of the channel Several Different types are used that have much less resistance Source to Drain Types: Lateral Double Defused MOSFET (LDMOSFET)» Shorter Channel between Source and Drain Less Resistance» Thus higher currents

17 Power MOSFET Field Effect Transistors Several Different types (continued) Types: Vertical MOSFET or V-Channel MOSFET (VMOSFET)» HAS a shorter/wider channel» Lower resistance» Thus more current flows» Has two Source and a Gate connection on top» Drain on the bottom» Lower Capacatance TMOSFET» Similar to the VMOSFET» No V Channel

18 Power MOSFET Several Different types (continued) Types: TMOSFET» Gate is embedded in the Silicon Dioxide Layer» The contact from the gate is over a wider area than for the non-power MOSFETs» Has smaller physical size than VMOSFETs Key Characteristics Lower Source to Drain resistance Higher currents The text covers VMOSFET in more detail

19 Vertical MOSFET Becomes conductive with a + charge on the Gate Channel resistance increases with temperature, thus VMOSFETs don t succumb to thermal runaway Much higher current capacity than traditional MOSFETs Example: IRF-100 N-Channel MOSFET I D Max = 16A and typical transconductance of 3 mhos

20 VMOSFET Package & Curves IRF-100

21 Vertical MOSFET Symbol Same as for a plain MOSFET Like other MOSFETs these are susceptible to static discharges Many VMOSFETS have a Zener diode built in to protect the against static discharge Zeners act as a like an open circuit when reversed biased with an appropriate voltage If the gate becomes negative, the zener will conduct» Best to have a 1kΩ on gate

22 Vertical MOSFET Typical MOSFET Power Interface Figure 4-16 on page 81 Gate signals can come from a digital circuit, e.g., microcontrollers, microprocessors, TTL circuits L 1 is a solenoid coil D 1 and R 1 are used to prevent coil kickback voltage from growing high enough to damage Q 1 or other components Operation» Normally reversed biased when solenoid is energized» Kickback voltage can forward bias D 1 W/gate at a logic low (approx 0 VDC) Q 1 is off and L 1 is off W/gate at a logic level 1 (+5 VDC) Q 1 is on and L 1 is energized

23 Vertical MOSFET Troubleshooting Tips If a bad MOSFET is found, check before replacing the MOSFET and applying power Typical FET Circuits Q 9 on Next Slide and Figure 4-17 on page 82 Used as a Noise amplifier Biasing Current flow through Source resister R 142 Gate voltage held at 0 VDC by R 58 Input signal coupling Through C 112 and coil L 20» Higher impedance for higher frequeny

24 Courier Cruiser CB Circuit Typical FET Circuits Q 9 on Next Slide and Figure 4-17 on page 82 Output drives Q 10 (looks like Q 1 )

25 Typical FET Circuits Q 1 on Next Slide and Figure 4-18 on page 82 Dual Gate MOSFET used as a amplifier Biasing Positive voltage on G 2 provides biasing» Feed by AGC through R 49» When Input amplitude increases AGC voltage on G 2 decreases» When Input amplitude decreases AGC voltage on G 2 increases Biasing controls Q 1 gain» Q 1 Gain decreases when voltage on G 2 decreases» Q 1 Gain increases when voltage on G 2 increases Output signal stays constant with changes in Input

26 Courier Cruiser CB Circuit