Circuits that contain negative feedback loops, whether current or voltage loops, with loop gains near or above unity may exhibit undesirable anomalies in frequency response when the phase shift of the signal inside the loop is sufficient to convert negative into positive feedback. For the current feedback loop of the Wilson current mirror this effect appears as a strong broad resonant peak in the ratio of the output to input current, formula_74, at about formula_75. Gilbert shows a simulation of a Wilson current mirror implemented in NPN transistors with formula_76 GHz and current gain formula_77 that shows a peak of 7.5 dB formula_78 at 1.2 GHz. This behavior is very undesirable and can be largely eliminated by further modification of the basic mirror circuit. Figure 4b show a possible variant on the Wilson mirror that reduces this peak by disconnecting the bases of Q1 and Q2 from the collector of Q2 and adding a second emitter to Q3 to drive the bases of the internal mirror. For the same bias conditions and device type, this circuit exhibits flat frequency response to 50 MHz, has a peak response less than 0.7 dB formula_79 at 160 MHz and falls below its low-frequency response at 350 MHz.