Electronics Switches & Controls

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Delilah Sparks
5 years ago
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1 lectronics Switches & ontrols onventional controls Potentiometer switches Transistor as switches and controls FT-based JT-based

2 ariable potentiometers -ariable resistance sometime needed (see Labs) - potentiometer provides such a variable resistance. -Make sure the circuit works for any transitor from this batch.

3 lectronics Switches -Switches are elements with two states «Open» and «lose» -Simplest switches are mechanical switches

4 efresher on transistors -Transistors can be used as switches, to control resistance, etc JT FT = + NPN PNP S S G = S + G JFT P-channel G JFT N-channel

5 Operating mode a a NPN JT Active mode : 0. 7 ~ 0.3 < < c β Open mode : 0 0 Sat. mode : β c ~0.7 h F ~0.8 ~0.2 Active mode Open mode Saturated mode = external voltage source supplytin et. cannot be larger than cc!

6 Operating mode a a NPN JT Active mode : 0. 7 ~ 0.3 < < ( < 0) c β Open mode: 0 0 Sat. Mode: c β ~0.7 h F ~0.8 ~0.2 Active mode Open mode Saturated mode

7 JT as a switch + c t>0 : > ~0.8,so that c c ~ ~few 100m 0.7 t losed switch t<0 : < 0.7 Open mode + ~0.8 ~0.2 << Open switch = 0 = 0.2

8 JT as a switch cc c losed switch + c Open switch 0.7 t min (losed switch ) cc β c min 0.8

9 JT as a switch: turning on/off a light -JT with 5 voltage source, - esign a system to switch a 20 ma, 5 lamp on and off, - The gain factor for a given batch of transitor varies between 100<β<500, -Make sure the circuit works for any transitor from this batch. 1 1 i = ic max( i ) = ic max( i ) = 0. 2mA β min( β ) N = + i = = 10 = kω i 0.2 N 22

10 JT as an amplifier P. Piot, PHYS 375 Spring assumptions : Transistor operates in active mode when v = 0 v OUT Amplitude of v signal small enough to have active mode operation n 1 st approximation : i ( << ) = + c + v b Neglecting variation of : ic v Finally : + out = cc c = S vs with : S = cc and vs c cic = vb = Signal v is amplified by a factor Av = c

11 JT as an amplifier: light detection Previous circuit can be used to amplify low intensity signal, e.g. such as produced in photodiode (typically 1 µa/1µw of incident light) ircuit is modified as follows: + OUT Output voltage is given by: OUT = = P. Piot, PHYS 375 Spring 2008 From ground---ground loop: OUT Finally output voltage is: OUT = = = ( ) 0.7

12 Two-states electronics When in =0 transistor is open, out = 20 As in increases the transistor turns on (active mode ) and current begin to flow out is lowered toward ground. ventually ~ 0.8 (saturated mode), and when in ~ 1 c =β b ~ 20 ma so OUT is close to 0 This circuit has only two output states (with exception of 0.8 < in <1 ) Two-states electronics important components of digital electronics

13 FT as control (ma) = + S sat GS P GS =0 transistor everse biased S > Ssat SS GS =-1 GS () GSoff P S () Pinch-off regime for S > Ssat : 1 GS SS GS off 2 = k ( ) 2 GS GSoff k = SS 2 GS off Linear (Ohmic) regime for S < Ssat : 2k ( ) S GS GS S off 2

14 FT as control When GS GSoff, the channel is depleted transistor is blocked When GSoff < GS< 0, and S > GS+P, (with P~ GSoff ), saturate and its value is quadractically dependent upon GS. P is the value of S for which saturates and GS is zero. ohmic Pinch-off breakdown The pich-off regime occurs for S= GS+P (GS is non zero) For 0 < S<< GS+P, is proportional to S (linear regime) and the slope is smaller as GS tends toward GSoff.

15 FT as resistance control For GS > GSoff and S < GS + P : example: S k 1 2 ( + ) S GS P v in v out v out S = v + S in = variable attenuation controlled by con hoosing >> S on, v out varies between ~0 et v in con Problem: S depends on S nonlinear response

16 FT as resistance control: a better solution S k ( + ) S GS 1 P 2 v in 1 v out S GS = com ( G 0) S k 1 ( + ) com P con 1 etter linearity than circuit in previous slide

17 FT as resistance control: variable frequency filter variably controlled resistance in an low (or high) pass filter ynamical adjustment of cut off frequency v in con 1 v out S k( com + P ) 1 1 ω S 1

18 FT switch based on MOFST ased on a voltage divider v OUT = S 47kΩ + 47kΩ v in When gate at ground S Ω v 0 > OUT When gate at +15 S 100 Ω v OUT v in

19 FT switch based on MOFST: example of application input output Selection of cut-off frequency

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