Transister Amplifire

Amplifier is a circuit that is used for amplifying a signal. The input signal to an amplifier will be a current or voltage and the output will be an amplified version of the input signal. An amplifier circuit which is purely based on a transistor or transistors is called a transistor amplifier. Transistors amplifiers are commonly used in applications like RF (radio frequency), audio, OFC (optic fibre communication) etc. Anyway the most common application we see in our day to day life is the usage of transistor as an audio amplifier. As you know there are three transistor configurations that are used commonly i.e. common base (CB), common collector (CC) and common emitter (CE). In common base configuration has a gain less than unity and common collector configuration (emitter follower) has a gain almost equal to unity). Common emitter follower has a gain that is positive and greater than unity. So, common emitter configuration is most commonly used in audio amplifier applications.

A good transistor amplifier must have the following parameters; high input impedance, high band width, high gain, high slew rate, high linearity, high efficiency, high stability etc. The above given parameters are explained in the next section.
Input impedance: Input impedance is the impedance seen by the input voltage source when it is connected to the input of the transistor amplifier. In order to prevent the transistor amplifier circuit from loading the input voltage source, the transistor amplifier circuit must have high input impedance.

Bandwidth:

The range of frequency that an amplifier can amplify properly is called the bandwidth of that particular amplifier. Usually the bandwidth is measured based on the half power points i.e. the points where the output power becomes half the peak output power in the frequency Vs output graph. In simple words, bandwidth is the difference between the lower and upper half power points. The band width of a good audio amplifier must be from 20 Hz to 20 KHz because that is the frequency range that is audible to the human ear. The frequency response of a single stage RC coupled transistor is shown in the figure below (Fig 3). Points tagged P1 and P2 are the lower and upper half power points respectively.

                                             RC coupled amplifier frequency response

Gain:

Gain of an amplifier is the ratio of output power to the input power. It represents how much an amplifier can amplify a given signal. Gain can be simply expressed in numbers or in decibel (dB). Gain in number is expressed by the equation G = Pout / Pin. In decibel the gain is expressed by the equation Gain in dB = 10 log (Pout / Pin). Here Pout is the power output and Pin is the power input. Gain can be also expressed in terms of output voltage / input voltage or output current / input current. Voltage gain in decibel can be expressed using the equation, Av in dB = 20 log ( Vout / Vin) and current gain in dB can be expressed using the equation Ai = 20 log (Iout / Iin).

Derivation of gain:
G = 10 log ( Pout / Pin)………(1)

Let Pout = Vout / Rout and Pin = Vin / Rin. Where Vout is the output voltage Vin is the input voltage, Pout is the output power, Pin is the input power, Rin is the input voltage and Rout is the output resistance. Substituting this in equation 1 we have
G = 10log ( Vout²/Rout) / (Vin²/Rin)………….(2)

Let Rout = Rin, then the equation 2 becomes
G = 10log ( Vout² / Vin² )
i.e.
G = 20 log ( Vout / Vin )

Efficiency:

Efficiency of an amplifier represents how efficiently the amplifier utilizes the power supply. In simple words it is a measure of how much power from the power supply is usefully converted to the output. Efficiency is usually expressed in percentage and the equation is  ζ = (Pout/ Ps) x 100. Where ζ is the efficiency, Pout is the power output and Ps is the power drawn from the power supply.

Class A transistor amplifiers have up to 25% efficiency, Class AB has up to 55% can class C has up to 90% efficiency. Class  A  provides excellent signal reproduction but the efficiency is very low  while Class C has high efficiency but the signal reproduction is bad. Class AB stands in between them and so it is used commonly in audio amplifier applications.

Stability:

Stability is the capacity of  an amplifier to resist oscillations. These oscillations may be high amplitude ones masking the useful signal or very low amplitude, high frequency oscillations in the spectrum. Usually stability problems occur during high frequency operations, close to 20KHz in case of audio amplifiers. Adding a Zobel network at the output, providing negative feedback etc improves the stability.

Slew rate:

Slew rat of an amplifier  is the maximum rate of change of output per unit time. It represents how quickly the output of an amplifier can change in response to the input. In simple words, it represents the speed of an amplifier. Slew rate is usually represented in V/μS and the equation is  SR = dVo/dt.

Linearity:

An amplifier is said to be linear if there is a linear relationship between the input power and the output power. It represents the flatness of the gain. 100% linearity is not possible practically as the amplifiers using active devices like BJTs , JFETs or MOSFETs  tend to lose gain at high frequencies due to internal parasitic capacitance. In addition to this the input DC decoupling capacitors (seen in almost all practical audio amplifier circuits) sets a lower cutoff frequency.

Noise:

Noise refers to unwanted and random disturbances in a signal. In simple words, it can be said to be unwanted fluctuation or frequencies present in a signal. It may be due to design flaws, component failures, external interference, due to the interaction of two or more signals present in a system, or by virtue of certain components used in the circuit.

Output voltage swing:

Output voltage swing is the maximum range up to which the output of an amplifier could swing. It is measured between the positive peak and negative peak and in  single supply amplifiers it is measured from positive peak to the ground. It usually depends on the factors like supply voltage, biasing, and component rating.

Common emitter RC coupled amplifier:

The common emitter RC coupled amplifier is one of the simplest and elementary transistor amplifier that can be made. Don’t expect much boom from this little circuit, the main purpose of this circuit is pre-amplification i.e to make weak signals strong enough for further processing or amplification. If designed properly, this amplifier can provide excellent signal characteristics. The circuit diagram of a single stage common emitter RC coupled amplifier using transistor is shown in Fig1.

                                                                 RC coupled amplifier

Capacitor Cin is the input DC decoupling capacitor which blocks any DC component if present in the input signal from reaching the Q1 base. If any external DC voltage reaches the base of Q1, it will alter the biasing conditions and affects the performance of the amplifier.

R1 and R2 are the biasing resistors. This network provides the transistor Q1′s base with the necessary bias voltage to drive it into the active region. The region of operation where the transistor is completely switched of is called cut-off region and the region of operation where the transistor is completely switched ON (like a closed switch) is called saturation region. The region in between cut-off and saturation is called active region. Refer Fig 2 for better understanding. For a transistor amplifier to function properly, it should operate in the active region. Let us consider this simple situation where there is no biasing for the transistor. As we all know, a silicon transistor requires 0.7 volts for switch ON and surely this 0.7 V will be taken from the input audio signal by the transistor. So all parts of there input wave form with amplitude ≤ 0.7V will be absent in the output waveform. In the other hand if the transistor is given with a heavy bias at the base ,it will enter into saturation (fully ON) and behaves like a closed switch so that any further change in the base current due to the input audio signal will not cause any change in the output. The voltage across collector and emitter will be 0.2V at this condition (Vce sat = 0.2V). That is why proper biasing is required for the proper operation of a transistor amplifier.

                                                                BJT output characteristics

Cout is the output DC decoupling capacitor. It prevents any DC voltage from entering into the succeeding stage from the present stage. If this capacitor is not used the output of the amplifier (Vout) will be clamped by the DC level present at the transistors collector.

Rc is the collector resistor and Re is the emitter resistor. Values of Rc and Re are so selected that 50% of Vcc gets dropped across the collector & emitter of the transistor.This is done to ensure that the operating point is positioned at the center of the load line. 40%  of Vcc is dropped across Rc and 10% of Vcc is dropped across Re. A higher voltage drop across Re will reduce the output voltage swing and so  it is a common practice to keep the voltage drop across Re = 10%Vcc . Ce is the emitter by-pass capacitor. At zero signal condition (i.e, no input) only the quiescent current (set by the biasing resistors R1 and R2 flows through the Re). This current is a direct current of magnitude few milli amperes and Ce does nothing. When input signal is applied, the transistor amplifies it and as a result a corresponding alternating current flows through the Re. The job of  Ce is to bypass this alternating component of  the emitter current. If Ce is not there , the entire emitter current will flow through Re and that causes a large voltage drop across it. This voltage drop gets added to the Vbe of the transistor and the bias settings will be altered. It reality, it is just like giving a heavy negative  feedback and so it drastically reduces the gain.

Design of RC coupled amplifier:

The design of a single stage RC coupled amplifier is shown below.
The nominal vale of collector current Ic and hfe can be obtained from the datasheet of the transistor.
Design of Re and Ce.
Let voltage across Re; VRe = 10%Vcc ………….(1)
Voltage across Rc; VRc = 40% Vcc. ……………..(2)
The remaining 50% will drop across the collector-emitter .
From (1) and (2)  Rc =0.4 (Vcc/Ic)  and Re = 01(Vcc/Ic).
Design of R1 and R2.
Base current Ib = Ic/hfe.
Let Ic ≈ Ie .
Let current through R1; IR1  = 10Ib.
Also voltage across R2 ; VR2 must be equal to Vbe + VRe. From this VR2 can be found.
There fore VR1 = Vcc-VR2. Since VR1 ,VR2 and IR1 are found we can find R1 and R2 using the following equations.
R1 = VR1/IR1 and R2 = VR2/IR1. 
Finding Ce.
Impedance of emitter by-pass capacitor should be one by tenth of Re.
i.e, XCe = 1/10 (Re) .
Also XCe = 1/2∏FCe.
F can be selected to be 100Hz.
From this Ce can be found.
Finding Cin.
Impedance of the input capacitor(Cin) should be one by tenth of the transistors input impedance (Rin).
i.e, XCin = 1/10 (Rin)
Rin = R1 parallel R2 parallel (1 + (hfe re))
re = 25mV/Ie.
Xcin = 1/2∏FCin.
From this Cin can be found.
Finding Cout.
Impedance of the output capacitor (Cout) must be one by tenth of the circuit’s output resistance (Rout).
i.e, XCout = 1/10 (Rout).
Rout = Rc.
XCout = 1/ 2∏FCout.
From this Cout can be found.
Setting the gain.
Introducing a suitable load resistor RL across the transistor’s collector and ground will set the gain. This is not shown in  Fig1.
Expression for the voltage gain (Av) of a common emitter transistor amplifier is as follows.
Av = -(rc/re)
re = 25mV/Ie
and rc = Rc parallel RL
From this RL can be found.

Current amplifier and buffers

Buffer amplifier:

Buffer amplifier is a circuit which transforms electrical impedance from one circuit to another.  The main purpose of a buffer is to prevent the loading of a preceding circuit by the succeeding one. For example, a sensor may have the capability to produce a voltage or current corresponding to a particular physical quantity it sense but it may not have the power to drive circuitry it is connected to. In such situations a buffer can be used. A buffer when connected between the sensor and the succeeding circuitry easily drives the circuitry in terms of current or voltage according to the sensor output.Buffers are classified into voltage buffers and current buffers. The symbols of ideal voltage buffer and current buffer are shown in Fig 1 and Fig 2 respectively.

Ideal voltage buffer symbol

Ideal current buffer symbol

Voltage buffer:

A circuit which transfers a voltage from a circuit with high output impedance to a circuit with low input impedance is call a voltage buffer. The voltage buffer connected between these two circuit prevents the low input impedance circuit ( second one) from loading the first one. Infinite input impedance, zero output impedance, absolute linearity, high speed etc are the features on an ideal voltage buffer.

If the voltage is transferred from the first circuit to the second circuit without any change in amplitude, then such a circuit is called unity gain voltage buffer or voltage follower. The output voltage just tracks or follows the input voltage. The voltage gain of the voltage follower is unity (Av = 1). Even though there is no voltage gain, there will be a sufficient amount of gain in current. So when a voltage follower is connected between two circuit, it will transfer the voltage from first one to second one without any change in amplitude and drives the second circuit without loading the first circuit.

A voltage buffer can be realized using opamp, BJT or MOSFET. Voltage follower using transistor (BJT) is shown in Fig 3. Voltage follower using BJT is also known as emitter follower. +Vcc is the transistor’s collector voltage, Vin is the input voltage, Vout is the output voltage and Re is the transistors emitter resistor.

Voltage follower implemented using opamp is shown in Fig 2. This is done by applying full series negative feedback to the opamp ie; by connecting the output pin to the inverting input pin. Here the opamp is configured in non inverting mode (refer Figure 2). So the equation for gain is Av= 1 + (Rf/R1).

Since output and inverting input are shorted ,Rf=0 .

Since there is no R1 to ground, it can be considered as an open circuit and so R1 = ∞

There fore (Rf/R1) = (0/∞)  = 0.

Therefore Voltage gain Av = 1 + (Rf/R1) = 1+0 =1.

Voltage follower using transistor

Voltage follower using opamp

Current buffer:

Current buffer is a circuit that is used to transfer current from a low input impedance circuit to a circuit having high input impedance. The current buffer circuit connected in between the two circuits prevents the second circuit from loading the first circuit. The features of an ideal current buffer are infinite input impedance, zero output impedance, high linearity and fast response. A current buffer with unity gain (B=1) is called a unity gain current buffer or current follower. Here the output current just tracks or follows the input current. A current buffer can be realised using transistor (BJT or MOSFET).

Current amplifier circuit:

A current amplifier circuit is a circuit which amplifies the input current by a fixed factor and feeds it to the succeeding circuit. A current amplifier is somewhat similar to a voltage buffer but the difference is that an ideal voltage buffer will try to deliver whatever current required by the load while keeping the input and output voltages same, where a current amplifier supplies the succeeding stage with a current that is a fixed multiple of the input current. A current amplifier can be realized using transistors.The schematic of a current amplifier circuit using transistors is shown in the figure below. Two transistors are used in this circuit.  β1 and  β2 are the current gains of transistors Q1 and Q2 respectively. Iin is the input current, Iout is the output current and+Vcc is the transistor T2′s collector voltage  The equation for the output current is Iout = β1 β2 Iin .

                                                         Current amplifier using transistors

12V battery level indicator circuit (LED bargraph)

The heart of this circuit is the LM3914 from national semiconductors. The LM3914 can sense voltage levels and can drive a display of 10 LEDs in dot mode or bar mode. The bar mode and dot mode can be externally set and more than one ICs can be cascaded together to gat an extended display. The IC can operate from a wide supply voltage (3V to 25V DC). The brightness of the LEDs can be programmed using an external resistor. The LED outputs of LM3914 are TTL and CMOS compatible.

Description.

In the circuit diagram LEDs D1 toD10 displays the level of the battery in either dot or bargraph mode. Resistor R4 connected between pins 6,7 and ground controls the brightness of the LEDs. Resistors R1 and POT R2 forms a voltage divider network and the POT R2 can be used for calibration.

The circuit shown here is designed in order to monitor between 10.5V to 15V DC. The calibration of the circuit can be done as follows. After setting up the circuit connect a 12V DC source to the input. Now adjust the 10K POT to get the LED10 glow (in dot mode) or LEDs up to 10 glow (in bar mode). Now decrease the voltage in steps and at 10.5 volts only LED1 will glow. Switch S1 can be used to select between dot mode and bar graph mode. When S1 is closed, pin9 of the IC gets connected to the positive supply and bar graph mode gets enabled. When switch S1 is open pin9 of the IC gets disconnected to the positive supply and the display goes to the dot mode.

With little modification the circuit can be used to monitor other voltage ranges. For this just remove the resistor R3 and connect the upper level voltage to the input. Now adjust the POT R2 until LED 10 glows (in dot mode). Remove the upper voltage level and connect the lower level to the input. Now connect a high value POT (say 500K) in the place of R3 and adjust it until LED1 alone glows. Now remove the POT, measure the current resistance across it and connect a resistor of the same value in the place of R3. The level monitor is ready.

Circuit diagram of battery level indicator using LM3914.

Battery level indicator circuit using LM3914

Cascading two LM3914.

Two or more LM3914 ICs can be cascaded together to get an extended display. The schematic of two LM3914 ICs cacaded together to get a 20 LED voltage level indicator is shown below.

Cascading two LM3914

Microprocessor and Microcontroller – The difference

When you start learning about Microprocessors (in most case you will begin with Intel 8085) and Microcontrollers (usually you will begin with Intel 8051 from the MCS 51 micro controller family), the first question that pops up is “hey… what’s the difference in between” ? In this article I am explaining the basic differences and similarities between a microprocessor and micro controller. In fact you can call this article a simple comparison of both micro computing devices. This comparison will be same (at the basic level) for any micro processor and controller.  So lets begin.

At the basic level, a microprocessor and micro controller exist for performing some operations – they are – fetching instructions from the memory and executing these instruction (arithmetic or logic operations) and the result of these executions are used to serve to output devices. Are you clear? Both devices are capable of continuously fetching instructions from memory and keep on executing these instructions as long as the power is not turned off. Instructions are  electronic instructions represented by a group of bits. These instructions are always fetched from their storage area, which is named as memory.  Now lets take a closer look at block diagrams of a microprocessor based system and a micro controller based system.

Microprocessor based system:

Take a closer look at the block diagram and you will see a micro processor has many support devices like Read only memory, Read-Write memory, Serial interface, Timer, Input/Output ports etc. All these support devices are interfaced to microprocessor via a system bus. So one point is clear now, all support devices in a microprocessor based system are external.  The system bus is composed of an address bus, data bus and control bus.

Okay, now lets take a look at the microcontroller.

Micro controller system:

The above block diagram shows a micro controller system in general. What’s the primary difference you see? All the support devices like Read only memory, Read – Write memory, Timer, Serial interface, I/O ports are internal. There is no need of interfacing these support devices and this saves a lot of time for the individual who creates the system. You got the basic understanding ? A micro controller is nothing but a microprocessor system with all support devices integrated inside a single chip. There is no need of any external interfacing in a micro controller unless you desire to create something beyond the limit, like interfacing an external memory or DAC/ADC unit etc. To make this microcontroller function, you need to give a DC power supply, a reset circuit and a quartz crystal (system clock) from external source.

Okay, so we have an idea about the basic difference between a microprocessor and microcontroller. Now lets compare some features of both systems.

Comparison:

As you already know, support devices are external in a microprocessor based system where as support devices are internal for a micro controller. Micro controllers offer software protection where as micro processor base system fails to offer a protection system. This is made possible in microcontrollers by locking the on-chip program memory which makes it impossible to read using an external circuit. Okay! So that are basic differences, now you can come up with some more. As we need to interface support devices externally in a microprocessor based system, time required to build the circuit will be more, the size will be more and power consumption will be more in a microprocessor based system compared to microcontroller.

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