simple adaptive bias amplifier, finalized!

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Ami
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Joined: Fri Sep 14, 2018 6:06 pm

#1 simple adaptive bias amplifier, finalized!

Post by Ami »

Hello Peep's.

The following is the project that the electronics class were asked to present after the Autumn term, the beginner forum of this board contains some of the meanderings and prevarication that went on before my final design was settled. I'm posting this to save the confusion that I caused there, and to thank those who offered help, everyone's comments were appreciated, especially those who directed me to gain some background knowledge on walking before trying to run :D

Here's the assignment.
.................

The term practical work project is to research and build a design for a functional radio receiver, the requirement for the completed set is that it can resolve local stations transmitting on the medium wave band with good selectivity and clarity, the test of sensitivity will be its ability to receive a 1KW station on 648KHz that is 100Km distant without any external antenna The design can be original or preexistent but the student must have built it and be able to demonstrate understanding of its function.

The technical specifications of the receiver are that it:

1. Should be transportable and self contained.

2. Be powered by primary cells, at a nominal maximum of 3volts.

3. Will include no more than 3 active devices. They will be bipolar transistors, and can be selected from types BC327, BC337, BC549, BC559, in any combination.

4. Will drive a loudspeaker so it can be used by multiple listeners.

5. Incorporate in its design an automatic method by which its battery power consumption is demonstrably economised, at low sound volume.

Marking will be by grade average and will be judged by:

6. Sensitivity.

7. Selectivity, its ability to separate signals.

8. Sound quality.

9. Maximum achievable sound level meter results before the onset of output clipping.

10. Economy, cost of battery cell's divided by hours running time, at a specific "volume" (Extrapolated from consumption and manufacturer data on cells capacity).
….…………..............

The following is an extract from the overview that I wrote on the design. I've only included the audio amplifier part of it, as it was that that prompted me to join here in the first place, seeking knowledge and sweet audio wisdom.

....I've left out most of the contentious techno bullshit and references that the teach insisted that I include in the written work: I'm sure no serious audio DIY'er needs some kid spoonfeeding them electronics 101.

Here it is....

....with regards to the audio stage, it would be necessary to realize it with just two transistors.

To that end the "Sliding junior" amplifier (Radio Constructor magazine, June 1973), designed by Sir Douglas Hall was investigated, (see first attachment).
sliding junior.jpg
This design is based upon the idea that the input signal is AC coupled to a diode clamp circuit that has the effect of superimposing the signal on the amplifier bias so as to increase it for the transistors in sympathy with the instantaneous input level. The author claimed that this "feed forward sliding bias" arrangement ensured that there was always sufficient bias to keep the amplifier in a linear (class A) part of its transfer curve, without the high dissipation normally associated with fixed class A topologies, whilst also providing better sound quality than a class B amplifier at low volume settings.

Although the design did work when built near faithful to the original as was possible, the claims of the author proved to be inaccurate, as the design suffered from an unusual distortion that is a product of the gain being dependant on the current through the output transistor, controlled by the instantaneous input level.

Another drawback noted was that on occasions the design was unable to stay fully in class A operation. This was found to be due to a transient lack of bias, sounding to the listener much like cross over distortion in the class B amplifier over which the author claimed his design had superiority. The reasons for this turned out to be quite complex and lay with the nature of the input signals. The amplifier performance was adequate on signals that comprised complex high energy content such as orchestral music, but was unsatisfactory with low energy, or discontinuous content, for instance speech or solo guitar. This was also more noticeable with increased impedance of the signal source output, and it would have been necessary to include a follower stage at the input, increasing the transistor count.

The last flaw in the design that was addressed was the inclusion of an output transformer, this component was assumed to be included to provide coupling to the speaker without the latter needing to pass the standing DC current of the output transistor. As a feature, this component was historically also associated with an impedance matching role, though in a design of this supply voltage, it isn't necessary, it also introduces losses and distortion. DC blocking by a large value capacitor being a suitable alternative. The load for the output transistor remained inductive in the final version however as it suited the requirements of maximum achievable efficiency and output power with the limited supply voltage and restrictions on active component count.

The solution to the drawbacks noted was the adoption of another method of providing adaptive bias in sliding bias amplifiers that Sir Douglas Hall dismissed as inferior. In early incarnations of this topology, a proportion of the amplifier output is fed back via a diode/capacitor combination providing a DC level that is then used to bias the output stage of the amplifier.

A major difficulty with this method it that the smoothing capacitor's value is a compromise between being sufficiently large so as to prevent unwanted feedback of the audio signal, and small enough that the circuit's transient response prevents noticeable distortion due to insufficient bias. The suboptimal performance that was a result of this tradeoff contributed to the topology's unpopularity as they couldn't be expected to deliver more than "public address" standards of fidelity.

The final version of this design instead feeds back the audio as an AC signal through a capacitor, and adds its negative transitions to that set by the idle current potentiometer. This bootstrapping action effectively increases the input impedance and provides bias drive that up to the circuit limitations, keeps the amplifier working in its most linear region. In listening tests, there was also no discernable lag in transient response. The only adjustment of this dynamic bias needed is the choosing of a suitable value capacitor, however unlike the previous method mentioned, it seems in no way critical. In this example, a value of 0.47uF provided barely sufficient bias, whereas increasing to 10uF began to cause unnecessarily high dissipation. The final circuit uses a component of 1uF.

The topology in the final version to my knowledge has been tried by two others, (note 1 & 2), who confirmed that its transient response is superior to any other sliding bias amplifier that has anywhere close as low a component count.

A further improvement in audio quality and thermal stability was gained from including a 0.33ohm degeneration resistance. This resistor provides a degree of DC negative feedback, (approximately 10% of output) that improves gain linearity as it progressively removes bias from the BC 559 base with increased output stage current, this also aids the thermal stability.

The final version of this circuit is shown in the second circuit diagram. A copy of it was assembled by my generous Mentor, Norman (note 1) who also wound inductors for me.
slider amp v2.01.JPG
slider amp v2.01.JPG (18.18 KiB) Viewed 5324 times
It should be noted that this amplifier is deliberately sub optimal regarding output power, mainly because of the output transistor that had to be used (600mW max total dissipation), it also follows that the distortion figures are taken well away from the onset of output clipping. The frequency response is also limited because of its intended use, (AM radio), it was found that restricting this made for a more pleasant experience as it attenuated the level of the annoying heterodyne whistles that plague AM radio.

Although this amplifier topology will in all honesty never be worthy of the title HiFi, it does sound quite respectable. After the current project, I would like to investigate the possibility of a higher powered version, just to see if it could be done.

Norman also supplied the following figures that are results from his testing the amplifier.

At 1KHz.

Output power = 120mW RMS into 3R at an input of 100mV and supply of 3volts.

THD = 3% at 120mW

Frequency response = 120Hz - 8KHz +/- 3db

Efficiency = approximately 30% (supply current of 130mA at 3volts full output)

Note 1. Thanks to Mr Norman Preston, who worked for HMV, S.Smith Accessories, (Radiomobile Ltd, London) during the nineteen fifties and sixties as a car radio designer.

Note 2. Thanks to Mr Don Field, RF technician and technology journalist.

Its been emotional!
Luv,
Ami.
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