# Is this a current mirror?

I saw this in AN-19 of linear's application notes for a boost converter. It is an error amplifier for a boost converter.

I'm not quite sure what the symbol for Q55 and Q56. My best guess is a current mirror. Am I correct?

• Not 100% sure of this, but I interpret these as PNP transistors with two collectors, one of them connected to base (to ensure bias?) – Vicente Cunha May 15 '18 at 23:02

Q55 and Q56 are transistors that have multiple collector terminals, where one of the collectors is shorted directly to the base contact.

This keeps the transistor from fully saturating, similar to how a Schottky transistor works in 74S and 74LS type logic. Keeping the transistors out of saturation improves the performance of the circuit.

Not current mirrors. An example of a current mirror is Q60 and Q61.

• I was going to reference patent 5,317,208. But you have this. – jonk May 15 '18 at 23:03
• Q55 and Q56 DO act as current mirrors - even according to your description. According to the AN-19 they also amplify by a factor of 4. – Kevin White May 16 '18 at 0:13
• Q55 and Q56 are current mirrors only if their construction emulates Q60 and Q61 together. Normally the space is used to draw conventional current mirrors so we play on the same field. – Sparky256 May 16 '18 at 1:58
• I thought they were trying to save on drawing space, but thanks for the clarification. – Voltage Spike May 16 '18 at 4:58
• @davetweed Can you give a reference to this. As I understand it, a baker clamp with schottky diodes requires a dedicated mask step to create the series collector-base schottky. Is there some other way to effectively raise the voltage of one collector, so that this is not a current mirror – Henry Crun May 16 '18 at 23:13

Yes.

Hans Camerzind writes in Designing Analog Chips, on pg 3.3:

There is also a lateral PNP equivalent. Using a split collector this current mirror needs only a single device. Each collector being smaller, the maximum current is limited (100uA)

So this is a smaller way to implement a high side current mirror.

On the high side lateral PNPs can have 2 or more split collectors and on the low side, you can have multiple identical emitters on the same base. Both these can be used to give current mirrors with current ratios of 1:N