Systems Approach Electronics, Lecture notes of Electronics

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EFFECTS OF R

S

AND R

L Systems Approach

S L  Packaged Networks and Systems – increasing interest to systems approach to design and analysis  IC Package – contains stable, reliable, self-testing, sophisticated designs that would be bulky with discrete components  Systems Approach – not a difficult one to apply once the basic definitions of the various parameters are correctly understood and the manner in which they are utilized is clearly demonstrated

SYSTEMS APPROACH- EFFECTS OF R

S

AND R

L  w/ V i

Z

TH

= Z

O

= R

O  E TH

• open-circuit voltage between the output terminals identified as VO E TH

= A

VNL

V

i  A i is seldom part of the two-port system analysis of the active devices A i

= - A

V

( Z

i

/R

L

S L  Thevenin’s Equivalent Circuit

S L  Answer: Will assume that all biasing resistors are part of the no-load gain and that a loaded system requires an additional load RL connected to the output terminals.

S L  Operational (op-amp) Amplifier

S L  Applying a load to a two-port system: The loaded voltage gain of an amplifier is always less than the no-load level.

S L  The current gain is determined by:  the formula does not include the input impedance or current gain although the level of Ri may change with configuration The current gain can be obtained from the voltage gain and impedance parameters Z i and R L .

S L A: recall: Z i = 1.071 k, Z o = 3 k  A VNL

B: re model:

S L  any networks can be solved using the re model approach  the advantage of the systems approach is that once the two-port parameters of a system are known, the effect of changing the load can be determined directly  “package” approach – the developing trend  system – two-port parameters are provided/ know how to utilize a given data

S L  dc analysis: R L did not contribute to the dc load line  ac analysis: load and collector resistance in parallel  ac and dc: pass through the same Q-point – ensure a common solution for the network under ac or dc condition  For a particular design, the smaller the level of R L , the lower the level of ac voltage gain.

S L  Effect of the Source Impedance (R S

The parameters Zi and AVNL of a two-port system are unaffected by the internal resistance of the applied source. The output impedance may be affected by the magnitude of RS. For a particular amplifier, the larger the internal resistance of a signal source, the less the overall gain of the system.

S L  Example:

S L A: two-port system B: re model