Answer MET Question 41
Question: Differentiate with the aid of simple sketches between the following types of electronic circuits;
1. Rectifier circuit; 2. Amplifier circuit; 3. Oscillator circuit.
Where $\displaystyle \small \mathrm{L=L_1+L_2+2M}$ , output is taken from another coil coupled to $\displaystyle \small \mathrm{L_1\ or\ L_2}$
Answer: 1. Rectifier circuit:
The diagram above shows a full wave rectifier circuit with its output wave form
Simply
defined, rectification is the conversion of alternating current (AC) to
direct current (DC). This involves a device that only allows one-way
flow of electric charge, this is exactly what a semiconductor diode
does.
Rectifiers can be of half wave and full wave type.
Bridge Rectifier Circuit in full wave rectifier configuration has shown in the figure. Also called full-Wave Bridge Rectifiers, it is built around a four-diode bridge configuration. Regardless of the polarity of the input, the current flows in the same direction through the load. That is, the negative half-cycle of source is a positive half-cycle at the load.
The bridge rectifier consisting of four diodes enables full wave rectification without the need for a centre tapped transformer.
The bridge rectifier is an electronic component that is widely used to provide full wave rectification and it is possibly the most widely used circuit for this application. Using four diodes the bridge rectifier the circuit has a distinctive format with the circuit diagram based on a square with one diode on each leg.
In most power supply applications, the output from a bridge rectifier will be connected to a smoothing capacitor as part of the load. These electronic components accept charge during the high voltage parts of the waveform and then give out charge to the load as the voltage falls. In this way they provide a more constant voltage than the direct output from the bridge rectifier.
In terms of the bridge rectifier and its diodes, the inclusion of the capacitor means that the current taken through the diodes will have significant peaks as the capacitor charges up. When selecting the electronic components for the bridge rectifier, it is necessary to ensure that they can accommodate the peak current levels.
There are several points that need to be considered when using a bridge rectifier to provide a DC output from an AC input:
Voltage drops: As most bridge rectifiers use silicon diodes, this drop will be a minimum of 1.2 volts and will increase as the current increases. Accordingly the maximum voltage output that can be achieved is a minimum of 1.2 volts down on the peak voltage of the AC input.
Heat dissipation: The voltage drop and the current passing through the rectifier will give rise to heat which will need to be dissipated. In some instances this can be easily dissipated by air cooling, but in other instances, the bridge rectifier may need to be bolted to a heat sink. Many bridge rectifiers are constructed to be bolted onto a heat sink for this purpose.
Peak inverse voltage: It is very important to ensure that the peak inverse voltage of the bridge rectifier, or individual diodes is not exceeded otherwise the diodes could break down. The PIV rating of the diodes in a bridge rectifier is less than that required for the two diode configuration used with a centre tapped transformer. If the diode drop is neglected, the bridge rectifier requires diodes with half the PIV rating of those in a centre-tapped rectifier for the same output voltage. This can be another advantage of using this configuration.
Rectifiers can be of half wave and full wave type.
Bridge Rectifier Circuit in full wave rectifier configuration has shown in the figure. Also called full-Wave Bridge Rectifiers, it is built around a four-diode bridge configuration. Regardless of the polarity of the input, the current flows in the same direction through the load. That is, the negative half-cycle of source is a positive half-cycle at the load.
The bridge rectifier consisting of four diodes enables full wave rectification without the need for a centre tapped transformer.
The bridge rectifier is an electronic component that is widely used to provide full wave rectification and it is possibly the most widely used circuit for this application. Using four diodes the bridge rectifier the circuit has a distinctive format with the circuit diagram based on a square with one diode on each leg.
In most power supply applications, the output from a bridge rectifier will be connected to a smoothing capacitor as part of the load. These electronic components accept charge during the high voltage parts of the waveform and then give out charge to the load as the voltage falls. In this way they provide a more constant voltage than the direct output from the bridge rectifier.
In terms of the bridge rectifier and its diodes, the inclusion of the capacitor means that the current taken through the diodes will have significant peaks as the capacitor charges up. When selecting the electronic components for the bridge rectifier, it is necessary to ensure that they can accommodate the peak current levels.
There are several points that need to be considered when using a bridge rectifier to provide a DC output from an AC input:
Voltage drops: As most bridge rectifiers use silicon diodes, this drop will be a minimum of 1.2 volts and will increase as the current increases. Accordingly the maximum voltage output that can be achieved is a minimum of 1.2 volts down on the peak voltage of the AC input.
Heat dissipation: The voltage drop and the current passing through the rectifier will give rise to heat which will need to be dissipated. In some instances this can be easily dissipated by air cooling, but in other instances, the bridge rectifier may need to be bolted to a heat sink. Many bridge rectifiers are constructed to be bolted onto a heat sink for this purpose.
Peak inverse voltage: It is very important to ensure that the peak inverse voltage of the bridge rectifier, or individual diodes is not exceeded otherwise the diodes could break down. The PIV rating of the diodes in a bridge rectifier is less than that required for the two diode configuration used with a centre tapped transformer. If the diode drop is neglected, the bridge rectifier requires diodes with half the PIV rating of those in a centre-tapped rectifier for the same output voltage. This can be another advantage of using this configuration.
2. Amplifier circuit
The diagram above shows an actual transistor amplifier circuit
A
transistor amplifier operate using AC signal input which alternate
between a positive value and a negative value so some way of
“presetting” the amplifier circuit to operate between these two maximum
or peak values is required. This is achieved using a process known as
Biasing. Biasing is very important in amplifier design as it establishes
the correct operating point of the transistor amplifier ready to
receive signals, thereby reducing any distortion to the output signal.
The Common Emitter Amplifier Circuit
The single stage common emitter amplifier circuit shown above uses what is commonly called “Voltage Divider Biasing”. This type of biasing arrangement uses two resistors as a potential divider network across the supply with their center point supplying the required Base bias voltage to the transistor. Voltage divider biasing is commonly used in the design of bipolar transistor amplifier circuits.
voltage divider network
This method of biasing the transistor greatly reduces the effects of varying Beta, ( β ) by holding the Base bias at a constant steady voltage level allowing for best stability. The quiescent Base voltage (Vb) is determined by the potential divider network formed by the two resistors, $\displaystyle \small \mathrm{R_1, R_2}$ and the power supply voltage Vcc as shown with the current flowing through both resistors.
Then the total resistance RT will be equal to $\displaystyle \small \mathrm{R_1+ R_2}$ giving the current as i = Vcc/RT. The voltage level generated at the junction of resistors $\displaystyle \small \mathrm{R_1\ and R_2}$ holds the Base voltage ($\displaystyle \small \mathrm{V_b}$ ) constant at a value below the supply voltage.
Then the potential divider network used in the common emitter amplifier circuit divides the supply voltage in proportion to the resistance. This bias reference voltage can be easily calculated using the simple voltage divider formula below:
Transistor Bias Voltage
$\displaystyle \small \mathrm{V_B = \frac{V_{cc}R_2}{R_1+R_2}}$
The same supply voltage, ($\displaystyle \small \mathrm{V_{cc}}$ ) also determines the maximum Collector current, $\displaystyle \small \mathrm{I_c}$ when the transistor is switched fully “ON” (saturation), $\displaystyle \small \mathrm{V_{ce}=0}$ . The Base current Ib for the transistor is found from the Collector current, $\displaystyle \small \mathrm{I_c}$ and the DC current gain Beta, β of the transistor.
Beta Value
$\displaystyle \small \mathrm{\beta =\frac{\Delta I_C}{\Delta I_B}}$
Beta is sometimes referred to as $\displaystyle \small \mathrm{h_{FE}}$ which is the transistors forward current gain in the common emitter configuration. Beta has no units as it is a fixed ratio of the two currents, $\displaystyle \small \mathrm{I_c}$ and $\displaystyle \small \mathrm{I_b}$ so a small change in the Base current will cause a large change in the Collector current.
The Common Emitter Amplifier Circuit
The single stage common emitter amplifier circuit shown above uses what is commonly called “Voltage Divider Biasing”. This type of biasing arrangement uses two resistors as a potential divider network across the supply with their center point supplying the required Base bias voltage to the transistor. Voltage divider biasing is commonly used in the design of bipolar transistor amplifier circuits.
voltage divider network
This method of biasing the transistor greatly reduces the effects of varying Beta, ( β ) by holding the Base bias at a constant steady voltage level allowing for best stability. The quiescent Base voltage (Vb) is determined by the potential divider network formed by the two resistors, $\displaystyle \small \mathrm{R_1, R_2}$ and the power supply voltage Vcc as shown with the current flowing through both resistors.
Then the total resistance RT will be equal to $\displaystyle \small \mathrm{R_1+ R_2}$ giving the current as i = Vcc/RT. The voltage level generated at the junction of resistors $\displaystyle \small \mathrm{R_1\ and R_2}$ holds the Base voltage ($\displaystyle \small \mathrm{V_b}$ ) constant at a value below the supply voltage.
Then the potential divider network used in the common emitter amplifier circuit divides the supply voltage in proportion to the resistance. This bias reference voltage can be easily calculated using the simple voltage divider formula below:
Transistor Bias Voltage
$\displaystyle \small \mathrm{V_B = \frac{V_{cc}R_2}{R_1+R_2}}$
The same supply voltage, ($\displaystyle \small \mathrm{V_{cc}}$ ) also determines the maximum Collector current, $\displaystyle \small \mathrm{I_c}$ when the transistor is switched fully “ON” (saturation), $\displaystyle \small \mathrm{V_{ce}=0}$ . The Base current Ib for the transistor is found from the Collector current, $\displaystyle \small \mathrm{I_c}$ and the DC current gain Beta, β of the transistor.
Beta Value
$\displaystyle \small \mathrm{\beta =\frac{\Delta I_C}{\Delta I_B}}$
Beta is sometimes referred to as $\displaystyle \small \mathrm{h_{FE}}$ which is the transistors forward current gain in the common emitter configuration. Beta has no units as it is a fixed ratio of the two currents, $\displaystyle \small \mathrm{I_c}$ and $\displaystyle \small \mathrm{I_b}$ so a small change in the Base current will cause a large change in the Collector current.
3. Oscillator circuit
The diagram above shows a Hartley Oscillator
Oscillators
are electronic circuits that generate a continuous periodic waveform at
a precise frequency. Two parts of the inductive coil are inductively
coupled and forms oscillation with the capacitor C. When the switch S is
closed and $\displaystyle \small \mathrm{V_{cc}}$ comes on, an
oscillation takes place with frequency, $\displaystyle \small
\mathrm{f_o=\frac{1}{2\pi \sqrt{LC}}}$ .Where $\displaystyle \small \mathrm{L=L_1+L_2+2M}$ , output is taken from another coil coupled to $\displaystyle \small \mathrm{L_1\ or\ L_2}$
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