Home > Articles > Output impedance of a jfet amplifier

Output impedance of a jfet amplifier

In electronics , a common-source amplifier is one of three basic single-stage field-effect transistor FET amplifier topologies, typically used as a voltage or transconductance amplifier. The easiest way to tell if a FET is common source, common drain , or common gate is to examine where the signal enters and leaves. The remaining terminal is what is known as "common". In this example, the signal enters the gate, and exits the drain.

===

We are searching data for your request:

Schemes, reference books, datasheets:
Price lists, prices:
Discussions, articles, manuals:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.
Content:
WATCH RELATED VIDEO: Parameters of JFET - AC Drain Resistance - Transconductance - Amplification Factor

Chapter 9 FET and MOSFET Amplifiers – Electronic Circuit Analysis


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:.


Lab 5 - JFET Circuits II

In CE arrangement, the value of input impedance is approximately equal to Increase in collector emitter voltage from 5V to 8V causes increase in collector current from 5mA to 5. Determine the dynamic output resistance. If source resistance in an amplifier circuit is zero, then voltage gain output to input voltage ratio and source voltage gain output to source voltage ratio are the same. Given that capacitance w. Which of the following generate output voltage much closer to true value? Guidelines All form fieds are required. Use your real email address as we will be sending you an email when someone replies to your comment.

The JFET can be used as a linear amplifier by reverse-biasing its gate relative of input impedance of this circuit is to be maintained, the output must.

The common-source JFET amplifier has:


Definition : The JFET General Source amplifier uses junction field-effect transistors as the main active device providing high input impedance characteristics. Transistor amplifier circuits e. This device has the advantage of high input impedance and low noise and is therefore very well suited for use in amplifier circuits with very small input signals. It is used in the class A amplifier circuit that we saw in the previous tutorial. These three JFET amplifier configurations support a grounded emitter, a grounded emitter, and a grounded basic configuration of a bipolar transistor. The JFET voltage Vg gate is selected by a power distribution network set up with resistors R1 and R2 and is biased to operate within the same range as the working area of the bipolar transistor. Unlike bipolar transistor circuits, intersected FETs use very small gates, so the gate can be made as an open circuit. In that case, there is no need to add.

Module 4.3

output impedance of a jfet amplifier

We will see the circuit symbols, basic biasing condition, the V-I characteristics, a simple amplifier circuit and few applications. In BJT transistors the output current is controlled by the input current which is applied to the base, but in the FET transistors the output current is controlled by the input voltage applied to the gate terminal. In the FET transistors the output current passes between the drain and source terminals and this path is called channel and this channel may be made of either P-type or N-type semiconductor materials. In BJT transistor a small input current operates the large load, but in FET a small input voltage operates the large load at the output.

Options A.

In a common source JFET amplifier, the output voltage is


FET, also called unipolar transistor is a transistor used to control the electrical behaviour of a device. Hence FET is an ideal device for use in almost every application in which transistors can be used. FETs are widely used as input amplifiers in oscilloscopes, electronic voltmeters and other measuring and testing equipment because of their high input impedance. Noise is an undesirable disturbance super-imposed on a useful signal. Noise interferes with the information contained in the signal; the greater the noise, the less the information. For instance, the noise in radio-receivers develops crackling and hissing which sometimes completely masks the voice or music.

FET Circuit Configurations

In electronic circuits, amplifiers are used to increase the strength or amplitude of the input signal without any phase change and frequency. Amplifier circuits are made up of either FET Fied Effect Transistor or normal bipolar junction transistor -based on their 3 terminals. The advantage of amplifier circuit using FET over BJTs is used as small-signal amplifiers because they produce high input impedance, high voltage gain, and low noise in the input signal. FET is a voltage-controlled device with three terminals -source, drain, and gate. Based on these terminals, FET is divided into 3 amplifier configuration that corresponding to 3 configurations of Bipolar transistors.

The zener diodes have a voltage rating equivalent to the intended output Input impedance is high as compared with JFET, Power consumption is low so that.

FET applications

Each have their own characteristics of voltage and current gain as well as input and output impedance. The choice of the FET circuit configuration or topology is one of the key design parameters on which the overall circuit design is based. The terminology used for denoting the three basic FET configurations indicates the FET electrode that is common to both input and output circuits. This gives rise to the three terms: common gate, common drain and common source.

JFETs junction field effect transistors have been with us for many years now, and there was a time when there were many different types available, often with some very desirable characteristics. JFETs became readily available about 10 years after BJTs bipolar junction transistors , and were quickly adopted for applications that required high impedance inputs. BJTs require an input current to conduct, and that means that they also require current from the signal source to change their output current. While the current is usually very low, it does cause problems in some cases.

Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts.

Effective date : Year of fee payment : 4. Year of fee payment : 8. In a circuit in which a common-source junction field effect transistor JFET is cascoded with a JFET element, current diversion or division circuits are used to divert a majority of the current passing through the input amplifier stage so that it bypasses the cascode FET without compromising the primary circuit function. The bypassing function is achieved by a current mirror, a current mirror-like circuit, or similar devices such as current sources, current splitters and the like and the circuits may be ratioed to more precisely control the bypass current by the use of emitter area scaling, ratioed emitter degeneration resistors, or both.

The output impedance of the on-chip amplifier can be as much as 2Kohms. The preamplifier noise should be substantially less than the CCD on-chip amplifier noise and have a lower noise corner frequency. Monolithic FET input op-amps with acceptably low current noise and voltage noise exist, but so far they do not have fast enough settling times to operate at 2 us per pixel or less. For example, the OPA op-amp 4.




Comments: 2
Thanks! Your comment will appear after verification.
Add a comment

  1. Aashish

    Anyone who does not think about distant difficulties will certainly face near troubles ...

  2. Stedeman

    at home with a curious mind :)