The µA709 Opamp

The µA709
Not long after the µA702 a major IC op amp landmark came about, specifi cally the introduction of another Bob Widlar op amp for Fairchild in 1965, the µA709, The 7091 improved markedly on the 702; it had higher gain (45,000 or ~94 dB), greater input/output ranges (±10 V), lower input current (200 nA) and higher output current, and operated from symmetrical power supplies (±15 V). The 709 quickly became a standard, and was produced for decades. 


So universal was the 709 that it can be regarded as an IC op amp classic. Although the individual specifi cations were surpassed by many subsequent designs, the 709 remains a milestone, as the fi rst widely used monolithic IC opamp.

lm709 Block Diagram
Many design principles from the 702 were used again in the 709, such as the use of matched transistors, for the fi rst and second stages, and the logarithmic biased (delta-VBE) current source, Q10–Q11. There were also new wrinkles added. Because the 709 used what was basically an NPN IC process, Widlar resorted to some clever tricks to create PNP functions. He used a modifi ed NPN structure for two PNPs, the level shifter Q9 and the output PNP, Q13. The output stage operated class-B, with no Q13–Q14 bias.
Local feedback around this stage via R15 minimized deadzone.

Frequency compensation for the 709 was achieved with two RC networks, between pins 1–8, and pins 6–5.

LM709 pin outline

The associated network values could be changed for optimum ac response, using four networks for gains of 0 dB to 60 dB.

Although the 709 was a vast improvement over the 702, it still had quirks of its own, and these gave rise to application problems. For example, without some user-added series resistance, the output stage could blow out for sustained shorts. Many saw the frequency compensation scheme as diffi cult, plus it took up board space.

Also, the 709 could latch up whenever the input CM voltage rose high enough to saturate the input stage. And, excessive differential input voltages could blow out the input transistors. Although savvy users could work around these 709 application quirks, it sometimes took extra parts to do it. So, in one sense the above use-related issues served as a general lesson towards the necessity of bullet-proofing an IC op amp.  against various application stresses.

Keywords : LM709, LM709CN, Opamp, Amps, IC, Amplifier, Circuit, Develop, Electronic, Integrated
Writer : delon  |
19 Apr 2006 Wed   
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