Phase Inverters

Four types of phase inverters for audio amplifiers are described in:
"Boyce, W.F.; Roche, J.J.: Radio Data Book, Boland & Boyce, 1948, pp. 144 - 146"

RADIO DATA BOOK [See- 1 Sec. 1] BASIC CIRCUITS

PHASE INVERTERS

A number of different types of phase inverter circuits are used. Although they are the same in purpose, they differ considerably and are therefore treated here as separate circuits. Four representative types which work on different basic principles as follows are:

Fig. 149- 180 degree phase shift between grid and plate voltages of a tube.

Fig. 150- 180 degree phase shift between grid and screen voltages in a tube.

Fig. 151- 180 degree phase shift between grid and cathode voltages in a tube.

Fig. 152- 180 degree phase shift between the voltages at the ends of an inductor with center tap grounded.

It will be noted that resistance coupling is used in all of these circuits except the last which uses impedance coupling.

Phase inverters became practical with the advent of high gain amplifier tubes. Previously, the gain resulting from the transformer "step-up" was an important factor. It was impractical to substitute a phase inverter for a push-pull transformer because of the relative loss in gain. Modern high gain tubes make amplification in the coupling circuits unnecessary and allow full use of the excellent frequency response of resistance coupling.

Another feature of these circuits is that negative feedback can often be simultaneously introduced as an integral part of the inverter circuit. The diagram of Fig. 149 with the dash line connection and Figs, 150 and 151 all provide negative feedback as part of the inversion process.

Applications.- Phase inverter circuits are used in audio and video amplifiers in which a single-ended stage is to be coupled to a push-pull stage without the use of an interstage transformer. Most high fidelity audio amplifiers and public address systems use phase inverters. They are also used extensively in all types of receivers.

Advantages.- Advantages of phase inverters are:

1. They allow resistance coupling between single ended and push pull stages.

2. A large, heavy interstage transformer is replaced by a few small components.

3. Phase inverters are less expensive.

4. Negative feedback can be incorporated as part of the circuit.

Limitations.- Phase inverters must produce a balanced output to keep distortion to a minimum. The circuits are often critical in this respect and must be carefully designed.

Variations.- The four representative ones shown and several others working on the same principles.

Tube Types Used.- Any audio frequency amplifier tubes, except in Fig. 150, which requires screen grid tubes.

 

 

In the circuit of Fig. 149 the input signal is applied to the grid of V1. It is then amplified in the usual way and appears at the grid of V3. The grid resistor of V3 is divided into two parts. That part of the signal appearing across R is fed to the grid V2, then amplified and applied to the grid of V4. Since the grid voltage of V4 has passed through one more tube than the V3 grid voltage, the two voltages are 180 degrees out of phase as required.

 

The circuit of Fig. 150 applies the principle that the signal voltage appearing across an unbypassed screen resistor is 180 degrees out of phase with the grid voltage of the same tube.
The input voltage is applied to VI. The out-of-phase signal is fed from the screen of VI to the grid of V2.

PHASE INVERTER III

The circuit of Fig. 151 works on the principle that the unbypassed cathode signal voltage is in phase with the grid signal voltage. The ground end of R thus has a voltage with

respect to cathode which is out of phase with the grid voltage of V1. This out of phase voltage is effectively applied to the grid of V2 since this grid is grounded. The grid voltages are thus 180 degrees out of phase as required.

 

The circuit of Fig. 152 uses inductor L to produce the necessary 180 degree phase relation. The input signal voltage is amplified through V1 in the usual way, using portion AB of L as a load impedance. This signal then appears at the grid of V2. Signal fluctuations in AB induce voltages of opposite phase in the CB portion of L. This out-of-phase voltage is applied to the grid of V3, thus, accomplishing the required signal phase inversion between V2 and V3.

 

Phase inverter type 3 is of special interest.

Firstly, it can be found in several radio sets. And more interesting, it is related to a differential amplifier. Some of the sets therefore do not connect R to ground like in Fig. 151 but to the secondary of the PA transformer. With this connection, the floating of the cathodes, which are necessary for symmetric operation of the stage, can be provided. Well, this works for AC only. But that is sufficient for this purpose.

Regards,

Dietmar

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Another look to phase inverter type 3 shows the similarity to a differential amplifier. (Indeed, also the inverter of type 1, namely V1 & V2 show this similarity.)

In this graph, on the left side phase inverter type 3 is redrawn with penthodes. In the middle the same schematic is shown in a way like a usual differential amplifier.

However, this is not yet a "normal" differential amplifier, because R_k has a small value and is connected to ground. If the schematic be a differential amplifier, R_k should have a big value and be connected to - U_B.  Indeed, differential amplifiers do have a current source as is shown at the righthand graph. The constant current I₀ distributes equally to each valves R1, R2 with no input signal u₁.

Now look at an incremental change of the input signal, say u₁ + Δu. This will change the equilibrium of the cathode currents to I₀/2 + Δi and I₀/2 - Δi, respectively, and I₀ will remain constant. The change of the input voltage Δu will appear at the grids as Δu/ and -Δu/2, respectively. This is the phase splitting or inverting effect.

Phase inversion in the PA stage therefore need twice the voltage of a single stage.

A current source or a  resistor with a high value may not be applicated in a PA stage due to power dissipation or low efficiency, respectively. Therefore it is necessary to reduce the value of R_k to its "normal" figure. However, with a low value of R_k the symmetry in the currents does no longer  hold.

To cure this, a voltage u_sek from the secondary of the PA transformer  with appropriate value can be added as shown in the righthand schematic.

Pus-pull PA stages in this kind can be found in several top radio models in 1953/54. 

Regards,

Dietmar

To thank the Author because you find the post helpful or well done.