AC Negative Feedback OpAmp Circuits

A common problem encountered in the development of transistor amplifier circuits is unwanted oscillation resulting from parasitic capacitance and inductance forming a positive feedback loop from output to input. Often, these parasitic parameters are quite small (nanohenrys and picofarads), resulting in very high oscillation frequencies.

Another parasitic effect in transistor amplifier circuits is Miller-effect capacitance between the transistor terminals. For common-emitter circuits, the base-collector capacitance (C_BC) is especially troublesome because it introduces a feedback path for AC signals to travel directly from the output (collector terminal) to the input (base terminal).

Does this parasitic base-to-collector capacitance encourage or discourage high-frequency oscillations in a common-emitter amplifier circuit? Explain your answer.

A student connects a model CA3130 operational amplifier as a voltage follower (or voltage buffer), which is the simplest type of negative feedback op-amp circuit possible:

With the non-inverting input connected to ground (the midpoint in the split 6/-6 volt power supply), the student expects to measure 0 volts DC at the output of the op-amp. This is what the DC voltmeter registers, but when set to AC, it registers substantial AC voltage!

Now this is strange. How can a simple voltage buffer output alternating current when its input is grounded and the power supply is pure DC? Perplexed, the student asks the instructor for help. Öh,” the instructor says, “you need a compensation capacitor between pins 1 and 8.” What does the instructor mean by this cryptic suggestion?

Some operational amplifiers come equipped with compensation capacitors built inside. The classic 741 design is one such opamp:

Find the compensation capacitor in this schematic diagram, and identify how it provides frequency-dependent negative feedback within the opamp to reduce gain at high frequencies.