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Common Emitter Amplifier Maximum Efficiency of Class A Amplifiers

Common Emitter Amplifier Maximum
Efficiency of Class A Amplifiers
Transformer Coupled Loads
We discussed using transistors as switches in the last lecture. Amplifiers are another extremely important use for transistors. Two types of transistor amplifiers are used in the NorCal 40A:
1. Linear amplifier – Called a “class A” amplifier. The output signal is a very close replica of the input signal shape. In other words, the output is simply a scaled version of the input. The Driver Amplifier (Q6) is an example.
2. Saturating amplifier – The shape of the output signal may be very different from the input. Between these two waveforms, perhaps only the frequency is the same. Additional “signal conditioning” is usually incorporated. The Power Amplifier (Q7) is an example. Saturating amplifiers are often much more efficient than linear amplifiers in converting power from the dc source to the signal ac (i.e., RF). The tradeoff is distortion in the amplified signal.
Common Emitter (CE) Amplifier
An example of what can be a linear amplifier is the common emitter amplifier shown in

We will restrict ourselves for the time being to circuits of this type when Q remains entirely in the active region. Note that V bb
is the input bias (i.e, dc) voltage used to set this operational condition.
Let’s now develop a qualitative understanding of this amplifier. Assume that the input voltage Vo is proportional to cos(?t ):
1. As Vo ?, Ib ? which implies Ic( = ?Ib) ?. Hence, V c ?. The maximum V c will be just below V cc .
2. Conversely, as Vo ?, I b ? which implies Ic =(?Ib) ?. Hence, V c ?. The minimum V c will be just above 0 and before Q saturates. (Actually, this minimum V will be quite a bit above saturation since we will see distortion in V c as Q approaches saturation, even though it’s not “technically” saturated.) From this discussion we can sketch these voltages and the collector current. Since Vo and Vc are 180º out of phase:

and with Ic and Vc also 180º out of phase

A good question to ask yourself at this point is “Just how does a transistor circuit actually amplify the input signal?”
Maximum Efficiency of Class A Amplifiers
As mentioned at the beginning of this lecture, the class A (or linear) amplifier produces as an output signal that is simply a scaled version of the input. We stated that this amplifier is not as efficient as others. We will now compute this efficiency to (1) understand what “efficiency” means, and (2) compare this efficiency with other amplifier types. The efficiency ? of the amplifier is defined as

where P is the RMS (ac) output power and Po is the dc supply power. We’ll separately compute expressions for each of these terms:
1. Po – this is the power supplied by the dc source. We’ll ignore the power consumed in the base circuit of because it will often be small compared to the power consumed in the collector circuit. Consequently,
P 0 =V cc I 0
Here, V cc is the dc supply voltage, but what is Io? This is a bit tricky. From the last figure, note that I c is comprised of two parts:
(a) dc component, and
(b) ac component.
The ac component is useful as an amplified version of the input signal Vo. However, it is the time average value of Ic which is the needed dc current I 0 in the calculation of

This is the maximum dc power supplied by the bias.
2. P – this is the RMS power supplied by the ac part of Vc (and Ic). For sinusoidal voltages and currents with peak-topeak amplitudes V pp and I pp respectively,

is the RMS (effective) ac output power. In the case here for maximum output voltage and current
V pp=Vcc
And

So that

Now, substituting (9.5) and (9.4) into (9.1) we have

This ? = 25% is the maximum efficiency of a class A (linear) amplifier connected to a purely resistive load. (Why is this the maximum value?) Practically speaking, it is unusual to operate an amplifier at its maximum output voltage. Consequently, the usual efficiencies observed for class A amplifiers range from 10% to 20%. This also helps to keep the signal distortion low. Class A amplifiers are notoriously inefficient, but they can be very, very linear.
Power Flow in Class A Amplifiers with Resistive Loads
It is extremely insightful to calculate the “flow of power” in this amplifier, beginning from the dc source to the ac power (signal power) delivered to the resistive load. Specifically, power flows from the dc source to both the load and transistor in the form of dc power and ac power (again, ignoring the base circuit). Let’s calculate the maximum of all four of these quantities separately:
(a) DC load power. This is due to the time average values of V and I in R and has nothing to due with the time varying component. From Fig. 9.2b

(b) AC load power. We computed this earlier

(b) AC load power. We computed this earlier

(c) DC transistor power. P tdc can easily be computed by noting that in this CE configuration (Fig. 9.2a), the average V and I across and through Q are the same as for the resistor R. In other words, the dc powers are the same for these two components:

(d) AC transistor power. This power is given by the usual Expression

Interestingly, since V c and I c 180º out of phase this ac power will be negative:

What does this minus sign mean? Q produces the ac power for the load! Cool. These results from (a) through (d) can be arranged pictorially as shown in

Class A Amplifier with Transformer Coupled Load
As mentioned in the text, there are two major disadvantages of class A amplifiers with resistive loads:
1. Half of the power from the supply is consumed as dc power in the load resistor.
2. Some types of loads cannot be connected to this amplifier. For example, a second amplifying stage would have the base of the transistor connected where R is located. The ac voltage would be excessively large for direct connection (typically want ac voltages from 10 – 100 mV or so). An interesting variation of the class A amplifier and one that removes both of these problems is to use a transformer coupled load as shown in

The Driver Amplifier (Q6) in the NorCal 40A is an example of such a class A amplifier with transformer load. From this circuit, we can see immediately that there will no longer be any dc power consumed in R since dc does not couple through transformers.
Next, notice that the dc resistance between V cc and Q is (nearly) zero so that the average collector voltage V c will then be V cc , not V cc /2 as before. This is very important to understand! (We will see this again later in connection with “RF chokes.”) With the transformer-coupled load, the maximum V c and I L are now twice as large as with a resistive load:

Let’s evaluate the efficiency of this new design. First, the maximum dc supplied power Po is

where R? is the effective load resistance due to T given as R? = n2R and n is the turns ratio N p /N s Next, the maximum ac (RMS) output power is

But

And
Vpp = 2Vcc
Notice that V pp is now twice V cc . The ac output power is then

Using (9.9) and (9.11) we find that the maximum efficiency is

In other words, the maximum efficiency of the class A amplifier with transformer coupled resistive load is ? = 50%. This is twice the efficiency of a class A amplifier with a resistive load. This doubling of efficiency makes sense since we’ve eliminated the dc power to the resistive load. (See the power flow diagram.)


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