Radio Frequency Electronics

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Laurence Powell
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1 Radio Frequency Electronics Active Components II Harry Nyquist Born in 1889 in Sweden Received B.S. and M.S. from U. North Dakota Received Ph.D. from Yale Worked and Bell Laboratories for all of his career Known for Nyquist rate, Nyquist plots, Nyquist stability criterion, Johnson-Nyquist noise, Nyquist-Shannon Sampling Theorem, and others Died in 1976 Image from Wikipedia 1

2 Step Recovery Diodes With proper doping levels and doping profile, one can make the reverse recovery a very narrow pulse. From Linear Systems theory we know that narrow pulses have significant harmonics. This is used in Step Recovery Diodes to generate high frequencies. 2

3 Step-Recovery Diodes 3 mm Example of an SRD SRD in Housing 3

4 Comb Generators 4

5 SRD Frequency Multiplier Filters to remove all but the desired harmonics SRD Transmission Line 5

6 Diode parameters in SPICE 6

7 V BR C J0 I 0 R S V bi Diode parameters in SPICE 7

8 Junction gradient coefficient Energy Bandgap Diode parameters in SPICE 8

important RC too large => output will not change fast enough and

9 Detectors Amplitude-modulated (AM) signal Information RF Carrier Diode detector Detected signal Proper choice of RC constant important RC too large => output will not change fast enough and output will be distorted RC should be about 0.5 of carrier frequency 9

10 Crystal Sets 10

11 Junction Field Effect (JFET) Transistors 11

12 JFET Transistors JFETs have characteristics that make them useful in some RF circuits, often at the front end. 12

13 Basic JFET Structure n channel JFET symbol p-channel JFET symbol Note that the device is inherently symmetrical and in many (but not all) commercial JFETs one can interchanges the drain and source. 13

14 Water Hose Analogy Increasing reverse-bias at gate increases depletion region that extends into channel, which constricts flow of carriers (electrons) in channel 14

15 Equilibrium Small, V D (and V G = 0) Voltage drop across channel V D = 5 V (and V G = 0) Depletion region is different across channel and channel narrows. With large enough V D, channel pinches off. V DS = V Dsat Some current still flows Post pinch off 15

16 Small, V D (and V G = 0) p + n Depletion region is different across channel and channel narrows. p + n With large enough V D, channel pinches off. V DS = V Dsat Some current still flows V P = pinch-off voltage = V Dsat when V G = 0 16

17 JFET Characteristics Ohmic Region Saturation region At small values of V D or V DS the plot can be quite linear V P = 2.5 V Plots such as these (I D versus V D or V DS ) are called the output characteristics of the device. 17

JFET as a Constant Current Source In saturation, with

source current through the device is independent of the

Compliance voltage the smallest voltage across while it

18 JFET as a Constant Current Source In saturation, with fixed V G the device behaves like a constant current source current through the device is independent of the voltage across the device. Compliance voltage the smallest voltage across while it stall behaves as a current source. Compliance voltage for V G = 0 Compliance voltage for V G < 0 18

pinched off and limits further increase Here I D generates a voltage drop, so that V G < 0,

19 JFET as a Constant Current Source Here V G = 0 and we are on this part of the curve. At small load current the device looks like a resistor, but at large load currents the channel pinched off and limits further increase Here I D generates a voltage drop, so that V G < 0, which widens the depletion region. Simple short-circuit protection Thus, it pinches off at a lower current 19

JFET ac Response g m is the transconductance and

Similar to g m = 40I C for BJT, but, but 10-20

similar to r o for the BJT C gd and C gs models

20 JFET ac Response g m is the transconductance and is tied to the current-source nature of the JFET. Similar to g m = 40I C for BJT, but, but smaller g d models the channel conductance and is similar to r o for the BJT C gd and C gs models the junction capacitances. C gd and C gs are small, few pf. 20

21 JFET Model 21

22 JFET Transfer Characteristics I D = I DSS 1 V GS V P 2 Plots such as these (I D versus V G or V GS ) are called the transfer characteristics of the device. 22

23 Applications of JFETs Constant current generators. Used in ICs Current limiting devices. Board level and ICs. Input stages of amplifier for very high input resistance. Pinch resistors. Input stages of amplifiers with very low noise. Voltage-controllable resistors. Used variable gain amplifiers, automatic gain control AGC, stages, Wien bridge stabilization. No diffusion capacitance, so have good high frequency response. JFETS make good mixers 23

24 Production Spread From datasheet for 2N3070 Notice the very large spread in important parameters Notice the comparatively low transconductance. 1 ma has g m of about 40 ms 24

Since the gate current is very small, one can use large value resistors, preserving the high input impedance of the JFET.

25 JFET Self Biasing A very common method of biasing a JFET is called self-biasing and looks a bit unusual when compared to BJT and MOSFET biasing. Self biasing consist of forcing the JFET s gate to 0 V. Since the gate current is very small, one can use large value resistors, preserving the high input impedance of the JFET. It is not unusual to see values of several MΩ used From a bias perspective, the value of R G is in most cases non-critical Load line Slope is 1 R s I D = I DSS 1 V GS V P 2 I D -V GS relationship for n JFET V GS = I D R S V GS R s = I DSS 1 V GS V P 2 KVL around R S, Gate and R g Combine and solve for V GS 25

JFET Self Biasing Assume I DSS = 10 ma, and V P = 3 V, find V S and I D. Check with SPICE. Solution I D = I DSS 1 V GS V P 2 V GS = I D R S V GS 4,700 = 0.01 1 V GS 3 V GS = 3.

26 JFET Self Biasing Assume I DSS = 10 ma, and V P = 3 V, find V S and I D. Check with SPICE. Solution I D = I DSS 1 V GS V P 2 V GS = I D R S V GS 4,700 = V GS 3 V GS = V, V GS = 2.33 V I D = 8.2 ma, V S = 38.5 V, 2 I D = ma V S = 2.34 V SPICE Check For the JFET set SPICE Gives BETA = I DSS V 2 P = V TO = 3 I D = ma V s = V 26

27 JFET Self Biasing works but it is probably better to use a coupling capacitor. 27

28 28

Electronic PRINCIPLES

MALVINO & BATES Electronic PRINCIPLES SEVENTH EDITION Chapter 13 JFETs Topics Covered in Chapter 13 Basic ideas Drain curves Transconductance curve Biasing in the ohmic region Biasing in the active region