Introduction

The concept of negative resistance to explain sustained oscillation in certain types of tuned oscillator has been around for many years. In fact, it can be found in publications even prior to 1930 and I thought it might be an interesting subject to revive. In the original article in Amateur Radio, a few circuits were included which took us back into the electron tube era. For brevity these have been omitted from the following presentation.

A Concept

As a first idea, let's look at the circuit of Figure la showing a resistance R connected across a battery with an output voltage E. Current I flows around the circuit so that power is dissipated in the resistance equal to E x I. Conventional current flow defines that current flows from positive to negative through the resistance. However, the same current also flows through the battery but in the reverse direction, negative to positive. Now the resistance R is a consumer of power and R = E/l. On the other hand the battery, as a generator of power, is exhibiting a characteristic defined by -E/l = -R, that is, a value equal to that of the resistance load but reversed in sign.

We look further at the AC circuit (Figure 1b) in which the current through resistance R is in phase with the applied AC voltage E. Current through the generator, whilst equal in value to that through the resistance load, is 180 degrees out of phase with the voltage E . Again the generator characteristic is defined by a resistance equal to load resistance but reversed in sign. The instantaneous currents through the resistance load and through the generator, as a function of instantaneous AC voltage, can be plotted as shown in Figure 2. From this figure we see that the slope of the curve is positive for the resistance load but negative for the generator.

This leads us to the basis of what is defined as negative resistance. If the current versus voltage curve for a circuit has a negative slope, it looks like a resistance in reverse and is said to have negative resistance. As seen. from the previous paragraphs, it is the same sort of I vs E characteristic as a generator and a negative resistance circuit can be considered as a generator. Placed in parallel with a real, or positive resistance, it feeds energy to the positive resistance load.

       

Oscillatory Tuned Circuit

A tuned circuit is formed by the parallel connection of an inductor and a capacitor. If energised, the circuit oscillates at its natural resonant frequency but the intensity of oscillation decays as a function of time because of energy losses in the circuit. The loss can be represented by a resistance component either in series or parallel with the tuned circuit as shown in Figure 3. To maintain continuous oscillation, sufficient energy must be continuously fed into the tuned circuit to balance the energy lost. A typical electronic oscillator is formed by connecting the tuned circuit to the input of an amplifier and feeding part of the amplifier output signal back into the tuned circuit. This replaces the energy loss in the tuned circuit and hence maintains oscillation.

Feedback can be achieved by inductive coupling from a coupling coil or via a feedback tap on the tuned inductor. In doing this, a four terminal or three terminal coil assembly is required. However, a two terminal coil assembly is achieved by connecting the tuned circuit in parallel with a circuit arrangement which has a negative resistance characteristic. Energy lost in the tuned circuit is supplied by the negative resistance circuit. One might imagine the negative resistance as cancelling the positive loss resistance and, in effect, making the series loss resistance look like zero, or the shunt loss resistance look like infinity.

Omissions from Original Article

At this point, the original article goes on to describe negative resistance in the screen grid or tetrode valve, the Dynatron Oscillator, the 6A8 Negative Transconductance Oscillator and the Magnetron Oscillator which are relevant to negative resistance application in the electron tube era. These sections have been omitted in this rewrite for brevity.

The Tunnel Diode

By around 1960, the bipolar transistor had been well established as an amplifying device to replace the electron tube. We were then introduced to a new semi-conductor device called the tunnel diode. A normal semiconductor diode is characterised by low resistance in the forward direction and high resistance in the reverse direction up to a point where the junction reverse breakdown voltage is reached. By increasing greatly the impurity concentration in the PN junction, the reverse breakdown voltage is reduced to zero, the diode conducts in the reverse direction and a peculiar effect, called quantum mechanical tunnelling, occurs in the forward direction. This produces the forward current characteristic of Figure 8 which includes a significant section of curve with a negative resistance characteristic. An oscillator circuit can be simply created by DC biasing the diode to some point in the negative resistance region of the curve and connecting a tuned circuit

       

The value of negative resistance is determined by the slope of the curve and, for stable operation at the operating point, the bias supply must have a lower source resistance than the value of negative resistance. On the other hand, too low a source resistance can cause the diode to oscillate at a frequency determined by its inherent self inductance and capacitance. Hence the value of bias supply source resistance can be critical. Its value is represented by load line curve C in Figure 9. A circuit for a 100 MHz oscillator is shown in Figure 10. For the circuit to oscillate at the required frequency, the shunt resistance presented by the tuned circuit L-C1 at resonance must be not less than the value of negative resistance. The resistance presented by the tuned circuit is represented by load line D in Figure 9.

       

If the load resistance presented by the tuned circuit is less than the negative resistance, the circuit can also be made to operate as an amplifier. One might compare this to a regenerative amplifier where positive feedback is set to a level below the point of oscillation and the amplifier achieves increased gain resulting from the feedback. To operate as an amplifier, the input signal can be fed in series with the paralleled diode and LC combination and the output taken from the parallel circuit. Of course the input source resistance and output load resistance have to be carefully arranged to satisfy the criteria discussed above. A practical circuit designed to amplify at 145 MHz, taken from an early issue of Amateur Radio, is shown in Figure 11. The designer of this circuit claimed gains of 30 to 40 dB at 145 MHz. The tunnel diode as an amplifier is somewhat limited to RF work as its linearity is hardly satisfactory for linear service such as in audio amplification.

One difficulty with the tunnel diode is the low voltage required for its operation. The negative resistance region is around 50 to 300 millivolts, requiring a bias voltage in the order of, say, 100 millivolts. It is a very high frequency device with operation possible at 1000 MHz, or even higher.

Conclusion

A relevant theme to finalise this article was extracted from an early copy of Wireless World
. "There are two ways of regarding the action of any oscillator which uses a two terminal circuit to fix the operating frequency. One is to consider the passive network as a feedback element between the output and input terminals of a power amplifier The other is to picture the amplifier as a negative resistance coupled to the passive network."
What has been highlighted is the second way using the negative resistance concept.

Some of the discussion in the original article is of historic interest. The Dynatron oscillator, as we have known it, has been passed by with the phasing out of the electron tube. At the time the tunnel diode was first introduced, it was thought to have a bright future at microwave frequencies because of its ability to perform at these frequencies with low noise. As it has turned out, it also appears to have been surpassed by better microwave performing devices.

There are various transistor circuits which have been made to look like a negative resistance source and work using a two terminal tuned network. There are also various integrated circuit packages which incorporate a tuned oscillator by connecting a two terminal tuned network. These days we tend to consider such packages as a black box and don't worry too much about how the circuit works inside the box. If we did, we might see a circuit which we could regard as a negative resistance source.

An interesting application is the Lambda Negative Resistance circuit using an N Channel and a P Channel JFET interconnected. This circuit has been used to make a Dip Meter.(ref 10).

References

1. Fundamental properties of Thermionic Valves - Radio Receiving Unit 2 - Melbourne Technical College.

2. Radiotron Designers Handbook 1955 - Section 24.2 - Negative Transconductance Oscillators.

3. Admiralty Handbook of Wireless Telegraphy - Section K63-65 The Magnetron.

4 RSGB Amateur Radio Microwave Techniques - Magnetron Oscillator.

5. Weber, F Sven - Tunnel Diode Amplifier - Amateur Radio, July 1965.

6 . Watters & Claeys - The Tunnel Diode Story - Amateur Radio, October 1960.

7. Review - Bright Future for the Tunnel Diode - Radio, TV & Hobbies, May 1960.

8. Rowe, Jamieson - Understanding Tunnel Diodes - Radio, TV & Hobbies, February 1961.

9. Rowe, Jamieson - Practical Aspects of the Tunnel Diode - Radio, TV & Hobbies, September 1961.

10. Butler, Lloyd (VK5BR) -A Dip Meter Using the Lambda Negative Resistance Circuit - Amateur Radio, January 1997.