In this article we discuss the third and the final design which is called the common-collector configuration.
The image of this configuration is shown below using the standard current flow directions and voltage notations:
Why Common Collector is Used
The primary objective of BJT common collector configuration is for impedance-matching purposes in electronic circuits.
This is due to the fact that this configuration possesses a high input impedance and a low output impedance.
This feature is actually the opposite of the other two counterparts common-base an common-emitter configurations.
How it Works
From the figure above we can see that the load here is attached with the emitter pin of the transistor and the collector is connected to a common reference with respect to the base (input).
Meaning, the collector is common to both the input and the output load. In other words, the supply coming to the base and the collector both share the common polarity. Here, the base becomes the input and the emitter becomes the output.
It would be interesting to note that, although the configuration resembles our previous common-emitter configuration, the collector can be seen attached with the "Common Source".
With regards to the design features, we don't have to incorporate the set of common collector characteristics for establishing the circuit parameters.
For all practical implementations, the output characteristics of a common-collector configuration will be exact as attributed for the common-emitter
Therfeore, we can simply design it by using the characteristics employed for the common-emitter network.
For every common-collector configuration, the output characteristics are plotted by applying IE vs VEC for the available IB range of values.
This implies that the both common-emitter and common-collector have identical input current values.
For achieving the horizontal axis for a common-collector, we just need to change the polarity of collector-emitter voltage in a common-emitter characteristics.
Finally, you will see that there's hardly any difference in the vertical scale of a common-emitter IC, if this is interchanged with IE in a common-collector characteristics, (since ∝ ≅ 1 ).
While designing the input side, we can apply the common-emitter base characteristics in order to achieve the essential data.
Limits of Operation
For every BJT we have an operational region or limit of operation over its characteristics which indicate its maximum tolerable range and the point where the transistor can work with minimum distortions.
The following image shows how this is defined for BJT characteristics.
You will also find these limits of operation on all transistor datasheets.
A few of these limits of operation are easily understandable, for example we know what is maximum collector current (referred to as continuous collector current in datasheet), and maximum collector-to-emitter voltage (typically abbreviated as VCEO in datasheets).
For the BJT which is demonstrated in the above image, we find IC(max) is specified as 50mA and VCEO as 20V.
The vertical line drawn indicated as VCE(sat) on the characteristic , exhibits the minimum VCE which can be implemented without crossing the non-linear region, presented with the name saturation region.
The VCE(sat) specified for BJTs is normally around 0.3V.
The higheest possible dissipation level is calculated using the following formula:
In the above characteristic image, the assumed BJT's collector power dissipation is shown as 300mW.
Now the question is, what is the method through which we can plot the curve for the collector power dissipation, defined by the following specifications:
This implies that the product of VCE and IC must be equal to 300mW, at any point on the characteristics.
If suppose IC has a maximum value of 50mA, substituting this in the above equation gives us the following results:
The above results tells us that if IC = 50mA, then VCE will be 6V on the power dissipation curve, as proven in Fig 3.22.
Now if we pick VCE with the highest value of 20V, then the IC level will be as estimated below:
This establishes the second point over the power curve.
Now if we select a level of IC around the mid-way, let's say at 25mA, and apply it on the resultant level of VCE, then we get the following solution:
The same is proven in Fig 3.22 also.
The 3 points explained can be effectively applied for getting an approximate value of the actual curve. No doubt we can use more number of points for the estimation and get even better accuracy, nevertheless an approximate becomes just enough for most applications.
The area that can be seen below IC = ICEO is called the cut-off region. This region must not be reached to ensure a distortion free working of the BJT.
You will see many datasheets only providing the ICBO value. In such situations we can apply the formula
ICEO = βICBO. This will help us to get an approximate understanding regarding the cut-off level in the absence of the characteristic curves.
In cases where you are unable to access the characteristic curves from a given datasheet, it may be imperative for you to confirm that the values of IC, VCE, and their product VCE x IC remain within the range as specified in the following Eq 3.17.