EDITOR: B. SOMANATHAN
NAIR
III. COMMON BASE LC-COUPLED HARTLEY OSCILLATOR
In the previous blog,
we had discussed the designs of the RC-coupled
and LC-coupled common-emitter type Hartley
oscillator. In this blog, we discuss the designs of the common-base and common-collector
types of Hartley oscillator. Figure 7 shows the common-base type of Hartley oscillator. It can be seen that this
circuit is similar to the CE LC-coupled Hartley oscillator shown in Fig. 4 of
our previous blog. The major differences between the two circuits are:
· In
the CE type, the emitter is bypassed whereas in the CB type, the base is
bypassed.
· In
Fig. 4 of our previous blog, we find that the centre tap is connected to VCC. In Fig. 7, this is connected to the emitter through a feedback
coupling capacitor.
· In
the CE type, the top end of the inductor is connected to the base through the
coupling capacitor, and in the CB type, this point is connected to VCC.
1. SPECIFICATIONS
·
Output swing : 4.5 V (peak)
·
Frequency of oscillation : 1
MHz
·
Current swing : 1 mA
2. DESIGN PROCEDURE
Steps 1 to
7: Design of the Standard Amplifier and the B Network
Follow the steps
given in earlier sections and design the Standard Amplifier and the B network.
Step 8: Design
of the Bypass Capacitor
The bypass capacitor CB is designed to be 5 mF, as given in a previous section.
Step 9: Design
of the Feedback Coupling Capacitor
The feedback from
collector-to-emitter is made through a 0.01-μF coupling capacitor and a 100-kΩ
variable potentiometer, as shown in Fig. 8. By adjusting the 100-kΩ pot, we can
control the gain to get a pure sine wave. The completely designed amplifier is
shown in Fig. 8.
A common-collector amplifier (or emitter follower) is an amplifier whose
gain is less than unity. Yet, it can be used for producing oscillation in the
Hartley mode of operation. Figure 9 shows the common-collector type of
Hartley-oscillator configuration. The loss in the gain of the amplifier is
compensated for by the amplification obtained through the step-up transformer
action of inductors L1 and
L2. It may be remembered in this context that the
gain of the Hartley-oscillator amplifier is given by the expression AV = L1/L2.
By choosing the value of inductor L1 much greater than that of L2 we can achieve the desired
amplification for producing oscillations. The excess gain can be attenuated by
using pot RF in series
with feedback coupling capacitor CC2,
as shown in Fig. 9.
1. SPECIFICATIONS
· Output swing : 4.5 V (peak)
·
Frequency of
oscillation : 1 MHz
·
Current swing : 1 mA
2. DESIGN PROCEDURE
Steps 1 to
7: Design of the Amplifier
In this section, we
have to design an emitter follower. We can get an emitter-follower amplifier by
removing the collector resistor RC
and shorting the collector terminal directly to +VCC. This makes the collector at ground potential under
ac conditions (under ac conditions +VCC
is also at ground potential). The output is taken across the emitter resistor RE. The B network is then attached as shown in Fig. 9.
Step 8: Design
of the B Network
To obtain the values
of inductors L1 and L2, we use the formula for
gain, which states that AV
= L1/L2. Since our amplifier is an emitter follower, its gain
is less than unity. Hence to increase the gain above unity, we must choose L1 > L2. to a good guess, let us choose L1 = 10L2. We also have the relation
L = L1 + L2.
From an earlier computation, we got the value of L = 253 μH. Using the two equations, we find
11
L2 = 253 μH
Hence
L2
= 253/11 = 23 μH
And
L1 = 23x10 = 230 μH
The excess gain will be dropped in the
feedback resistor RF to
yield pure sine wave. The completely designed amplifier is shown in Fig. 10.
2. SOME PRACTICAL HINTS FOR THE GENERATION OF OSCILLATIONS
a.
First
construct the amplifier and check its DC conditions for proper operation. Ensure
that its operating point is in the middle of the active region.
b.
If the
amplifier is perfect, then connect the B
network. It must be remembered that fresh and perfectly working components are
used in constructing the B network.
If both the above conditions are satisfied, then the oscillator will definitely
produce oscillations.
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ReplyDeleteI did an ngspice simulation of this oscillator. It worked with an ideal BJT model, but it wouldn't start with a realistic 2N2222A model until I raised the bias resistors. I wound up getting good results using 100k for the upper one (instead of 68k) and 22k for the lower one (instead of 12k). I also raised the emitter resistor to 4.7k. I think this may have reduced the amplitude of the oscillation, but it resulted in a much better sine wave.
ReplyDeleteFor anyone interested in trying this, note that in ngspice there must be a DC path to ground for all nodes, so a load resistor to ground needs to be added after the output capacitor. I used 30k.