1. Transistor

4. BJT Transistors: NPN Transistor PNP Transistor Sandwiching a P-type layer between two n- type layers. Sandwiching a N-type layer between two p- type layers.

5. How a “NPN” Transistor works? Forwardbackward The base-emitter diode (forward) acts as a switch. when v1>0.7 it lets the electrons flow toward collector. so we can control our output current (Ic) with the input current (Ib) by using transistors. CB E

6. Collector Emitter Base Transistors have three terminals: Transistors work in 3 regions Active: Always on Ic=BIb Saturation :Ic=Isaturation On as a switch Off :Ic=0 Off as a switch

7. Transistor as a Switch Transistors can be used as switches. 1 Transistors can either conductnot conduct conduct or not conduct current. 2 onoffie, transistors can either be on or off. 2 TransistorSwitch

8. Transistor Switching Example 15 When V BE is less than 0.7V the transistor is off and the lamp does not light. When V BE is greater than 0.7V the transistor is on and the lamp lights. X Variable Voltage Supply 12V

9. Transistor Circuit : Light-Controlled Circuit This transistor circuit contains a Light-Dependent Resistor. Because of the LDR, this circuit is dependent on light. The purpose of this circuit is to turn on the LED when the light reaches a certain intensity. Input= Voltage Divider Process= Transistor Output= LED 1)LED = Off. 2)Cover LDR. 3)R LDR . 4)V LDR . 5)Transistor switches on. 6)LED = On.

10. Transistor as an amplifier : Transistors are often used as amplifiers to increase input signal in radios, televisions and some other applications.The circuit may be designed to increase the current or voltage level. The power gain is the product of current gain and voltage gain (P=V*I).

11. Amplifier example: As you see, the transistor is biased to be always on. The input signal is amplified by this circuit. The frequency of output is the same as its input, but the polarity of the signal is inverted. The measure of amplification is the gain of transistor. Example: Input Amplitude =0.2v Output amplitude=10v Gain=10/0.2=50

12. Field Effect Transistors JFETMOSFETCMOS

13. When the gate is negative,it repels the electron in the N- channel. So there is no way for electrons to flow from source to drain. When the negative voltage is removed from Gate,the electrons can flow freely from source to drain.so the transistor is on. How a JFET transistor works?

14. When the Gate is positive voltage,it allows electrons to flow from drain to source.In this case transistor is on. In MosFET, the Gate is insulated from p-channel or n-channel. This prevents gate current from flowing, reducing power usage. How a MOSFET Transistor works?

15. How a CMOS transistor works? When Gate (input) is high,electrons can flow in N-channel easily. So output becomes low. (opposite of input) When Gate (input) is low,holes can flow in P-channel easily. So output becomes high. (opposite of input) N-channel & P-channel MOSFETs can be combined in pairs with a common gate.

16. Opamp

17. Schematic diagram of lm741

20. Ideal Opamp

21. Operational Amplifier (OP AMP) Basic and most common circuit building device. Ideally, 1.No current can enter terminals V + or V -. Called infinite input impedance. 2.V out =A(V + - V - ) with A → ∞ 3.In a circuit V + is forced equal to V -. This is the virtual ground property 4.An opamp needs two voltages to power it V cc and -V ee. These are called the rails. A Vo = (A V -A V ) = A (V - V ) + + - -

22. INPUT IMPEDANCE WHY? For an instrument the Z IN should be very high (ideally infinity) so it does not divert any current from the input to itself even if the input has very high resistance. e.g. an opamp taking input from a microelectrode. Input Circuit Output Impedance between input terminals = input impedance

23. OUTPUT IMPEDANCE Input Circuit Output Impedance between output terminals = output impedance WHY? For an instrument the Z OUT should be very low (ideally zero) so it can supply output even to very low resistive loads and not expend most of it on itself. e.g. a power opamp driving a motor

24. OPAMP: COMPARATOR V out =A(V in – V ref ) If V in >V ref, V out = +∞ but practically hits +ve power supply = V cc If V in <V ref, V out = -∞ but practically hits –ve power supply = -V ee V cc -V ee V IN V REF Application: detection of QRS complex in ECG A (gain) very high

25. OPAMP: ANALYSIS The key to op amp analysis is simple 1.No current can enter op amp input terminals. => Because of infinite input impedance 2.The +ve and –ve (non-inverting and inverting) inputs are forced to be at the same potential. => Because of infinite open loop gain 3.These property is called “virtual ground” 4.Use the ideal op amp property in all your analyses

26. OPAMP: VOLTAGE FOLLOWER V + = V IN. By virtual ground, V - = V + Thus V out = V - = V + = V IN !!!! So what’s the point ? The point is, due to the infinite input impedance of an op amp, no current at all can be drawn from the circuit before V IN. Thus this part is effectively isolated. Very useful for interfacing to high impedance sensors such as microelectrode, microphone…

27. OPAMP: INVERTING AMPLIFIER 1.V - = V + 2.As V + = 0, V - = 0 3.As no current can enter V - and from Kirchoff’s Ist law, I 1 =I 2. 4. I 1 = (V IN - V - )/R 1 = V IN /R 1 5. I 2 = (0 - V OUT )/R 2 = -V OUT /R 2 => V OUT = -I 2 R 2 6. From 3 and 6, V OUT = -I 2 R 2 = -I 1 R 2 = -V IN R 2 /R 1 7. Therefore V OUT = (-R 2 /R 1 )V IN

28. OPAMP: NON – INVERTING AMPLIFIER 1.V - = V + 2.As V + = V IN, V - = V IN 3.As no current can enter V - and from Kirchoff’s Ist law, I 1 =I 2. 4. I 1 = V IN /R 1 5. I 2 = (V OUT - V IN )/R 2 => V OUT = V IN + I 2 R 2 6. V OUT = I 1 R 1 + I 2 R 2 = (R 1 +R 2 )I 1 = (R 1 +R 2 )V IN /R 1 7. Therefore V OUT = (1 + R 2 /R 1 )V IN

29. SUMMING AMPLIFIER V OUT = -R f (V 1 /R 1 + V 2 /R 2 + … + V n /R n ) IfIf Recall inverting amplifier and I f = I 1 + I 2 + … + I n Summing amplifier is a good example of analog circuits serving as analog computing amplifiers (analog computers)! Note: analog circuits can add, subtract, multiply/divide (using logarithmic components, differentiate and integrate – in real time and continuously.

30. DRIVING OPAMPS For certain applications (e.g. driving a motor or a speaker), the amplifier needs to supply high current. Opamps can’t handle this so we modify them thus Irrespective of the opamp circuit, the small current it sources can switch ON the BJT giving orders of magnitude higher current in the load.