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Graham Slee一篇关于运放和分立晶体管电路的文章

(2013-05-12 22:26:29)





分类: 发烧理念

Op-Amps versus Discrete Transistors   运放和分立晶体管

25 February 2013 - by Graham Slee

Transistors are cheap. On average, small signal transistors are 2 cents (1.5 pence, 2.5 euro-cents) each. Production quantities are $20 per thousand - a pocket money investment.


Building hi-fi circuits using transistors is labour intensive but you sub it to a Chinese assembly shop and your circuit will often cost less than a single op-amp.


Bolt your Chinese made transistor boards in your Chinese made case and you can still announce your product is made in Britain, the USA, or wherever your company is.


Your Hi-Fi company is hungry for money so what explanation do you give your customers? You tell them how superior your discrete transistor circuits are - how they're superior to 'cheap' op-amps. You get your customers to look down their noses at op-amps and anybody who uses them - but in truth you're misleading people.


If you know anything about electronics at all, you know that discrete transistors have a wide matching tolerance which means the performance of your products will vary widely - unless you use lashings of negative feedback... but your ad says zero-negative feedback...


So you hand select the product you send for review. Your customers get what's left over...


Dear customer, are you still convinced discrete circuits are superior to op-amps?


In 1965 I had my first adventure in electronics - I realised how exact things had to be to work, never mind how well they worked. I was only 10 years old.


In a couple of year’s time my electronics experiences will have spanned 50 years! A lot of the companies tempting you for your money are operated by people with little or no experience in electronics (often a Chinese man does it all for them), yet they make incredible claims that sound so convincing - perhaps they're gifted in that way?


In these nearly 50 years I've witnessed the virtual end of the valve and the beginning of the 'tinny' germanium transistor. The improvement the silicon bipolar transistor made, the commercial use of field-effect-transistors (FETs and MOSFETs) and the introduction of operational amplifiers (op-amps).


You may find it surprising to know that the first ever op-amp was a circuit board containing quite a few valves (tubes). Silicon bipolar transistors later replaced the valves and the circuit boards of op-amps became smaller.


The performance breakthrough happened when mankind found how to etch and dope those transistors, passive components and interconnections onto a wafer of silicon. The modern hermetically sealed chip op-amp was born and with precision laser trimming repeatability of results was assured - a far cry from the matching difficulties you’d have with discrete transistors.


It also meant that wider bandwidths could be accommodated due to the reduction of circuit capacitance - much more stable (and hence reliable) than boards made with discrete transistors.


The op-amp gave real electronic designers the opportunity of making better circuits and in the case of high fidelity equipment, the awesome possibilities of incredible sound quality and amazing stereo imagery.


It is often a case that those who can - do, and those who cannot go into sales. There are a lot of 'cannots' in hi-fi judging by my experience.


One of the false sales claims from those 'who cannot', is that op-amps are ready-made amplifier stages whose performance is decided by the op-amp manufacturer. This is the farthest from the truth one can get!


They base the above argument on high school teaching which in recent years has seen the op-amp on the syllabus. High school subjects are nowhere near advanced and are extremely basic.


You can hazard a guess at the ages of these so called 'brilliant designer's' from what I just said. Their employers may be older and more experienced... but in sales!


There is more to designing with op-amps than most post graduates would have the time for. The amount of advanced educational material from op-amp manufacturers is colossal.


Apart from being superior in performance to an open (or even potted) board full of discrete transistors, the op-amp can be utilised in many different ways to those most students would know. The understanding of how op-amps work requires an advanced understanding of how discrete transistors work and that's just for the first course!


Often op-amps can be 'tamed' to have the same flat open-loop frequency response of valves (something I use in a number of my Ultra-Linear designs).


Most hi-fi designers have no knowledge that single op-amps have more than two inputs, let alone being able to tell the difference between noise gain and signal gain and its effect on high frequency stability - no wonder they hide behind their simplistic transistor designs.


But are they simplistic really? Many discrete transistor designs I've seen are direct copies of op-amp schematics! Op-amps are so bad they copy them???


I'll leave you with this thought: one major criticism levelled at op-amps is their internal circuit complexity. Out of the most 'complex' op-amp circuits I have ever seen, there are only five or six stages in the 'signal path' but most are four.


The simplest discrete transistor stage I have seen that can be considered to be compatible with input and output requirements has three stages, but mostly they're four, five or six!


The rest of the op-amp features these 'failed' designers cannot understand are the op-amp's power supply and power conditioning circuitry. Yes, they come with their own precision voltage references and power supply noise rejection circuits!


So what is the op-amp versus discrete transistor argument all about? Arrogance maybe?






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