BJT Differential Amplifier
Types
NPN 
DC Analysis(for desired operating point)
NPN Steps to Find the operating point 
PNP Steps to Find the Operating point
AC Analysis(obtaining voltage gain)
Differential Mode Gain and Common Mode Gain 
PNP 
To analysis its differential mode, the transistors are redrawn in equivalent t-model, and the following equations are obtained:
AC emitter resistance, ππ=π½π»/π°π¬ =ππππ½/π°π¬
Differential Mode gain, π¨π π=πΉπͺ/(ππ_π )
To analysis its common mode, it is obtained from βhalf circuitsβ:
Common Mode gain, π¨ππ=πΉπͺ/(ππΉ_π¬ )
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Common Mode Rejection Ratio 
"CMRR" ("dB" )=ππγ"log" γ"10" |π¨π π/π¨_ππ |
Thus, a large CMRR is desirable as it is less sensitive to noise. (CMRR>> 1). Since the CMRR can be a large number, it is often expressed in decibels or dB.
The main goal in circuit design is to minimize the noise level (or improve signal-to-noise ratio).
Thus CMRR shows how well the signal can be amplified and how well the noise is attenuated.
"CMRR"=|π¨π π/π¨ππ |
Differential mode gain, Adm to the common mode gain, Acm
Differential amplifier is a circuit that accepts two inputs and produces an output that is proportional to the difference between those inputs.
Differential Amplifier is used mainly to suppress noise and amplify signal.
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Operational Amplifier:
The standard op-amp symbol is shown above. The terminals are as follows:
V- : Input (Inverting)
V+ : Input (Non-Inverting)
Vo : Output
Vcc : DC Supply (positive)
VEE : DC Supply (negative or ground)
An op-amp is a high gain DC amplifier which can be used to amplify both DC and AC signals. It is made up of numerous transistors and other components like resistor and capacitor. 
Practical characteristics
Ideal characteristics
Infinite Open-Loop Voltage Gain (AOL)
Infinite Input Impedance, (ZIN)
Zero Output Impedance, (ZOUT)
Infinite Bandwidth, (BW)
Zero Output Offset
Finite gain (as large as possible)
Finite input impedance (as large as possible)
Non-zero output impedance (as small as possible)
Finite bandwidth (as large as possible)
Offset voltage and offset currents (as small as possible)
Open Loop Amplifier: The output of the open loop amplifier depends on :
The open loop gain of the amplifier, AOL.
The polarity relationship between V+ and V-.
The value of the supply voltages.
The load resistance
Working principle of amplifier as Voltage Comparator: 
Non-linear.
-Output will not increase or decrease proportionally with the input. For example, when the voltage is lesser than 0.7V. It can have little to no current. If the voltage is more than 0.7V, the current will increase depend on how the other components works within the circuit
Linear.
-Proportional or increase/decrease along the straight line. It exhibit behavior or response which are predictable.
-Example, V = IR
Where V is proportional to I * R. When R is constant, I goes when V also goes up.
Light Detecting Circuit 
Integrated circuit is referred as an IC, it is a set of electronic circuits on one small chip of semiconductor material. 
Non-linear and Linear
Discrete circuit is an electronic circuit built out of different discrete components. 
Can operate at higher voltages/deliver higher output current.
Can be customized for high performance.
Can be repaired piecemeal.
More reliable because of elimination of soldered joints and need for fewer interconnections.
Lower power consumption because of their smaller size.
Extremely small size β Thousands of times smaller than discrete circuits.
Very low cost because of simultaneous production of hundreds of similar circuits on a small semiconductor wafer.
Integrated and Discrete
Zero level detector 
Window Comparator Circuit :A window comparator circuit, also called window detector circuit or dual edge limit detector circuits is used to determine whether an unknown input is between two precise reference threshold voltages. It employs two comparators to detect over-voltage or under-voltage.
Connection to relay: This means that the relay can easily drive a high-power circuit using a low power circuit. This means that the relay has an input voltage of 12V DC. But it can easily drive a 240V circuit because of the phenomenon of electromagnetic induction turning the heater ON and OFF.
Light detecting circuit: The light sensing component of this circuit is a light detecting resistor, or LDR. This is just a photo-sensitive component that changes its resistance according to the light shining on it. The more light that shines on it, the lower is its resistance. 
