Why common collector amplifier is called as buffer

An amplifier is an electronic circuit that is used to amplify the voltage signal or a current signal. The amplifier circuits are generally designed with one or more transistors. They can be categorized into a weak signal amplifier or a power signal amplifier and are used in wireless communication and broadcasting, and audio equipment. This article discusses the common collector amplifier which is the amplifier topologies. The common collector amplifier is one of the three basic BJT amplifier topologies.

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Content:

• Common Collector Amplifier Circuit and Its Applications
• Current amplifier and buffers
• Transistor Emitter Follower Circuit: Common Collector Amplifier
• What is another name for a common collector amplifier?
• BJT Amplifier
• Transistors
• What is Common Collector Amplifier : Characteristics and Its Applications

WATCH RELATED VIDEO: Why Common Collector (CC) Amplifier is also called as Emitter Follower ?

Common Collector Amplifier Circuit and Its Applications

The term amplifier as used in this chapter means a circuit or stage using a single active device rather than a complete system such as an integrated circuit operational amplifier. An amplifier is a device for increasing the power of a signal. This is accomplished by taking energy from a power supply and controlling the output to duplicate the shape of the input signal but with a larger voltage or current amplitude.

In this sense, an amplifier may be thought of as modulating the voltage or current of the power supply to produce its output. The basic amplifier, figure 9. The transistor, as we have seen in the previous chapter, is a three-terminal device.

Representing the basic amplifier as a two port network as in figure 9. This means one of the transistor terminals must be common to both the input and output circuits.

This leads to the names common emitter, etc. The remaining terminal is what is thus common to both input and output. When larger multi-stage amplifiers are assembled, both types of transistors are often interspersed with each other. The base or gate terminal of the transistor serves as the input, the collector or drain is the output, and the emitter or source is common to both input and output it may be tied to the ground reference or the power supply rail , which gives rise to its common name.

The common emitter or source amplifier may be viewed as a transconductance amplifier i. As a transconductance amplifier, the small signal input voltage, v be for a BJT or v gs for a FET, times the device transconductance g m , modulates the amount of current flowing through the transistor, i c or i d.

By passing this varying current through the output load resistance, R L it will be converted back into a voltage V out. Nor is the output load, R L , low enough for a decent voltage amplifier ideally zero. More on how this capacitance effects the frequency response in a later section of this chapter.

Therefore, in practice the output often is routed through either a voltage follower common collector or drain stage , or a current follower common base or gate stage , to obtain more favorable output and frequency characteristics. This latter combination is called a cascode amplifier as we will see later in the chapter on multi-stage amplifiers. The generally lower g m of the FET vs. In order for the common emitter or source amplifier to provide the largest output voltage swing, the voltage at the Base or Gate terminal of the transistor is offset in such a way that the transistor is nominally operating halfway between its cut-off and saturation points.

This allows the amplifier stage to more accurately reproduce the positive and negative halves of the input signal superimposed upon the DC Bias voltage. Without this offsetting Bias Voltage only the positive half of the input waveform would be amplified. Figure 9. V DS curves and b I C vs. V CE curves.

The red line superimposed on the two sets of curves represents the DC load line of a ohm R L. To maximize the output swing it is desirable to set the operating point of the transistor, with a zero input signal, at a drain or collector voltage of one half the supply voltage, which would be 4 volts in this case.

Finding the corresponding drain or collector current along the load line gives us the target current level. This is around 10mA for R L equal to ohms. The I D equal to 10mA point on the load line falls between the 1.

The task now is to somehow provide this DC offset or bias at the Gate or Base of the transistor. The first bias technique we will explore is called voltage divider bias and is shown in figure 9. For the MOS case we know that no current flows into the gate so the simple voltage divider ratio can be used to pick R 1 and R 2. The actual values of R 1 and R 2 are not so important just their ratio. However, the divider ratio we choose will be correct for only one set of conditions of power supply voltage, transistor threshold voltage and transconductance, and temperature.

Actual designs often use more involved bias schemes. For the NPN case the calculation is somewhat more involved. We know we want I B to be equal to 50uA. The current that flows in R 1 is the sum of the current in R 2 and I B which puts an upper bound on R 1 when R 2 is infinite and no current flows in R 2. If we assume a nominal V BE of 0. To that end we need to make the current in R 2 many times larger than I B.

R 2 will be V BE divided by uA or 1. Taking I B into account shifted the required ratio. These values would need to be adjusted slightly if the actual V BE was not the 0. This points out a major limitation of this bias scheme as we pointed out in the MOS example above.

A consequence of including this bias scheme is a lowering of the input impedance. The input now includes the parallel combination of R 1 and R 2 across the input. For the MOS case this now sets the input resistance. There is another minor inconvenient problem with this bias scheme when it is connected to a prior stage in the signal path. This bias configuration places the AC input signal source directly in parallel with R 2 of the voltage divider.

This may not be acceptable, as the input source may tend to add or subtract from the DC voltage dropped across R 2. One way to make this scheme work, although it may not be obvious why it will work, is to place a coupling capacitor between the input voltage source and the voltage divider as in figure 9. The capacitor forms a high-pass filter between the input source and the DC voltage divider, passing almost the entire AC portion of the input signal on to the transistor while blocking all the DC bias voltage from being shorted through the input signal source.

This makes much more sense if you understand the superposition theorem and how it works. According to superposition, any linear, bilateral circuit can be analyzed in a piecemeal fashion by only considering one power source at a time, then algebraically adding the effects of all power sources to find the final result.

With only the AC signal source in effect, and a capacitor with an arbitrarily low impedance at the input signal frequency, almost all the AC voltage appears across R 2. To calculate the small signal voltage gain of the common emitter or source amplifier we need to insert a small signal model of the transistor into the circuit.

The following are some of the key model equations we will need to calculate the amplifier stage voltage gain. These equations are used for the other amplifier configurations that we will discuss in following sections as well.

The small signal voltage gain A v is the ratio of the input voltage to the output voltage:. The input voltage V in v be for the BJT and v gs for the MOS times the transconductance g m is equal to the small signal output current, i o in the collector or drain. V out will be simply this current times the load resistance R L, neglecting the small signal output resistance r o for the moment.

Notice the minus sign because of the direction of the current i o. Comparing these two gain equations we see that they both depend on the DC collector or drain currents. The Thermal Voltage, V T increases with increasing temperature so from the equation we see that the gain will actually decrease with increasing temperature. If R L is relatively large when compared to the small signal output resistance then the gain will be reduced because the actual output load is the parallel combination of R L and r o.

In fact r o puts an upper bound on the possible gain that can be achieved with a single transistor amplifier stage. Again looking at the small signal models in figure 9. For the MOS case V in will see basically an open circuit for low frequencies anyway.

This will of course be the case absent any Gate or Base bias circuitry. For most practical applications we can ignore r o because it is very often much larger than R L.

In applications where only a positive power supply voltage is provided some means of providing the necessary DC voltage level for the common gate or base terminal is required. This might be as simple as a voltage divider between ground and the supply. In applications where both positive and negative supply voltages are available, ground is a convenient node to use for the common gate or base terminal. The common gate or base stage is most often used in combination with the common emitter or source amplifier in what is known as the cascode configuration.

The cascode will be covered in the next chapter on multi stage amplifiers in greater detail. To calculate the small signal voltage gain of the common base or gate amplifier we insert the small signal model of the transistor into the circuit. It is perhaps more useful to consider the current gain of the current follower stage rather than its voltage gain. Thus the MOS stage current gain is exactly 1. The equation below from the BJT small signal T model relates g m and the resistance seen at the emitter r E.

We can also use this relationship to give us the resistance seen at the source r S. Thus the name current follower. We can generally assume this is true if we consider that V in is driven from a low impedance nearly ideal voltage source.

If this is not the case then the finite output impedance must be added in series with r o. If the input of the current follower is driven by the relatively high output impedance of a transconductance amplifier such as the common emitter or source amplifier from earlier then the output impedance for the combined amplifier can be very high.

The Emitter or Source follower is often called a common Collector or Drain amplifier because the collector or drain is common to both the input and the output.

This amplifier configuration, figure 9. The input to output offset is set by the V BE drop of about 0. The input impedance is much higher than its output impedance so that a signal source does not have to supply as much power to the input.

The low output impedance of the emitter follower matches a low impedance load and buffers the signal source from that low impedance. To calculate the small signal voltage gain of the voltage follower configuration we insert the small signal model of the transistor into the circuit. For the circuit in figure 9.

To use the voltage gain formula we just obtained using the small signal models we need to first calculate r E. From section 9. To use this formula we need to know I E. We know that the voltage across R L is V out. If we use an estimate of V BE to be 0. Substituting these values into our gain equation we get:.

Current amplifier and buffers

The amplifier is an electronic circuit that is used for amplifying a voltage or current signal. The input for the transistor will be a voltage or current and the output will be an amplified form of that input signal. An amplifier circuit is generally designed with one or more transistors is called a transistor amplifier. In this article, we will discuss the common-collector amplifier circuit. The Transistor amplifiers are most commonly using in our day to day life applications like an audio amplifier, Radio Frequency, audio tuners, Optical fiber communication , etc. As we discussed in our previous article, there are three transistor configurations that are used commonly for signal amplification i. Good transistor amplifiers essentially have the following parameters high gain, high input impedance, high bandwidth, high slew rate, high linearity, high efficiency, high stability, etc.

Transistor Emitter Follower Circuit: Common Collector Amplifier

In electronics, a common collector amplifier also known as an emitter follower or BJT voltage follower is one of three basic single stage bipolar junction transistor… … Wikipedia. In electronics, a common emitter amplifier is one of three basic single stage bipolar junction transistor BJT amplifier topologies, typically used as a voltage amplifier. Common drain — In electronics, a common drain amplifier, also known as a source follower, is one of three basic single stage field effect transistor FET amplifier topologies, typically used as a voltage buffer. In this circuit the gate terminal of the… … Wikipedia. In electronics, a common base also known as grounded base amplifier is one of three basic single stage bipolar junction transistor BJT amplifier topologies, typically used… … Wikipedia. Common gate — Figure 1: Basic N channel common gate circuit neglecting biasing details ; current source ID represents an active load; signal is applied at node Vin and output is taken from node Vout; output can be current or voltage In electronics, a common… … Wikipedia. Buffer amplifier — A buffer amplifier sometimes simply called a buffer is one that provides electrical impedance transformation from one circuit to another.

What is another name for a common collector amplifier?

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BJT Amplifier

Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up. Connect and share knowledge within a single location that is structured and easy to search. In common collector amplifier, we know that the collector is commons since it is connected to AC ground. Now, the characteristics of a common collector amplifier is that it has unity voltage gain, to be exact, it is less than 1 due to internal resistance of the transistor, and has high input impedance and low output impedance.

Transistors

Wiki User. Since its input impedance is much higher than its output impedance it is also termed a "BUFFER" for this reason it is also used in digital circuits with basic gates Common collector amplifier can be used as a voltage buffer and in impedance matching. A common-collector amplifier, also called an emitter follower, has the collector in both the input circuit and the output circuit. It is normally used as a buffer, because it has a high input impedance, low output impedance, a voltage gain of 1 but an appreciable power gain. Emitter followers can become unstable with a capacitve load, which can form an unwanted Colpitts oscillator with the stray capacitances.

What is Common Collector Amplifier : Characteristics and Its Applications

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This article deals with another type of bipolar transistor architecture used to amplify signals that is commonly known as Common Collector Amplifier CCA. The CCA can also sometimes be called emitter-follower amplifier and we will understand why later in this article. The first figure below is a simplified electric diagram with no particular biasing circuit presenting the CCA configuration :. We see in Figure 2 an equivalent circuit of the CCA configuration of Figure 1 considering the transistor such as described above. It is easy to understand that in the configuration presented in Figure 1 , the voltage gain is approximately equal to 1. To provide better stability, the base of the bipolar transistor is biased with a voltage divider network such as shown in the following figure.

In this configuration, the base terminal of the transistor serves as the input, the emitter terminal is the output and the collector terminal is common for both input and output. Hence, it is named as common collector configuration. The input is applied between the base and collector while the output is taken from the emitter and collector. In common collector configuration, the collector terminal is grounded so the common collector configuration is also known as grounded collector configuration. Sometimes common collector configuration is also referred to as emitter follower, voltage follower, common collector amplifier, CC amplifier, or CC configuration. This configuration is mostly used as a voltage buffer.

Our first article gave an introductory outline of bipolar transistor principles, characteristics, and basic circuit configurations. This time we'll concentrate on practical ways of using bipolar transistors in useful common-collector voltage follower circuit applications. The common-collector amplifier also known as the grounded-collector amplifier, emitter follower, or voltage follower can be used in a wide variety of digital and analog amplifier and constant-current generator applications.