Ideal Transistor Switch

A bipolar transistor can be made to approximate an ideal switch.
Consider a common emitter transistor circuit,

A collector resistance R_C is connected from the transistor collector to the supply voltage V_CC.
The emitter terminal of the device is grounded.
For the transistor to simulate a switch, the terminals of the switch are the transistor collector and emitter.
The input voltage or the controlling voltage for the transistor switch is the base-emitter voltage V_BE.
The collector-emitter voltage V_CE is equal to the supply voltage minus the voltage drop across R_C,

V_CE=V_CC-I_CR_C

When the transistor base-emitter voltage is zero or reverse biased, the base current I_B is zero, and the collector current I_C is also zero. The transistor switch is now in its OFF condition.

Since there is no collector current, there can be no voltage drop across the load resistor.
Therefore when I_C=0,

V_CE=V_CC

Thus, when an ideal transistor switch is OFF, its collector-emitter voltage equals the supply voltage.

When the transistor base is made positive with respect to the emitter, a base current I_B flows. The collector current I_C is equal to I_B multiplied by the transistor common-emitter dc current gain h_FE.
i.e.  I_C=h_FE*I_B 
If I_B is made large enough, I_CR_C can become equal to the supply voltage V_CC.

Therefore,

V_CE=V_CC-V_CC=0

Thus, when an ideal transistor switch is ON, its collector-emitter voltage equals zero.

Ideally, it dissipates zero power when ON or OFF.
Transistor power dissipation is given by,

P_D=I_CV_CE

When the switch is OFF, I_C=0,

P_D=0

When the switch is ON, V_CE=0,

P_D=0

The only time power is dissipated is when the device is switching between ON and OFF.

Communication Satellite Link Design

The main controlling factor in the design of links is the frequency of the uplink and downlink (6/4 GHz).

The optimum radio frequency range of satellite communications is determined technically by,

a) absorption, scattering and refraction phenomena rising in the atmosphere.

b) galactic noise and thermal noise emitted by the atmosphere.

c) the feasibility of constructing satellite antenna with gains appropriate to the service area required.

The best frequency range for systems serving fixed earth stations is about 1-10GHz.

The lower limit is set mainly by galactic noise and the size and mass of satellite antennas.

The upper limit is usually set by attenuation due to heavy rain.

In satellite communication link design the important calculation is the power received by the receiving station.

The transmitting source is considered to be located in free space and supposed to radiate the power P_T Watts uniformly in all directions. Normally a directive antenna is used in satellite communication link and the directivity is represented by a finite beam of width θ .

The receiver is characterized by the effective area A_R of its antenna and by the noise temperature T of its low noise amplifier.

The transmitting antenna beam illuminates an area A_O at the receiver.

The distance between transmitter and receiver is d.

Thus, the receiving antenna intercepts the fraction A_R/A_O of the transmitted power. Therefore, the power received by the receiving antenna is given by,

The directivity of antenna is described by its gain as,

which is actually the ratio of the area illuminated by an isotropic antenna to that illuminated by the antenna in question.

The product P_TG_T is called effective isotropic radiated power (EIRP).

The receiving antenna gain G_R is related to its effective area A_R by,

Therefore,


The power attenuation expressed in decibels is,


Thus,

where,

which is called as path loss or free space loss and expresses the signal power attenuation between two isotropic antenna in free space.

The free space loss varies with frequency, the higher the frequency, higher the free loss.

Path loss between a satellite in geostationary orbit and the earth station is 195.6dB and 199.1dB at 4GHz and 6GHz respectively.

In real sense there would be a variety of losses and so instead of L_FS it may be L such that L=L_FS*L_A

where L_A is additional losses given by,

where,
L_FTX = losses between the transmitter output and the transmitting antenna
A_AG = attenuation by the atmosphere and ionosphere
A_Rain = attenuation due to precipitation and clouds
L_POL = losses caused by polarization mismatch between the transmitter and receiver antenna
L_Point = losses caused by antenna depointing
L_FRX = losses between the receiving antenna and the receiver input

Therefore



The gain of an antenna is expressed in terms of actual surface area A by,

where η  is the efficiency of the antenna

The product ηA corresponds to the effective area A_R.

Normally the efficiency of 60% is taken as good though some antenna may achieve efficiency up to 70%.

SET-RESET (S-R) Flip-Flop

S-R flip-flop is an asynchronous sequential circuit.The S-R flip-flop has two inputs, namely SET(S) and RESET(R), and two outputs Q and Q̅.
The two outputs are complement to each other.

 The S-R flip-flop can be easily constructed using two NOR gates connected back to back.

The cross-coupled connections from the output of one gate to the input of the other gate constitute a feedback path.